## 1. The Moon Is a Round Sky Rock

### Topics

Astronomy; phases of the moon; shape of the moon; orbit of the moon; relationship of earth, moon, and sun; scientific inference; evidence

### Supplies

Each student needs a small sphere: berry or marble, or even a pea could work.  At my village I used mthula fruits.  It’s best if they can hold the sphere on a stick, so that it isn’t in the shadow of their hand holding it.

### Procedure

This must be done when the moon is visible in the daytime sky.  This means in the afternoon around first quarter and in the morning around third quarter.  (Around new moon, the moon is too close to the sun to be seen in the sky and around full moon, the moon is visible only at night, being above the horizon in daytime only in late evening before the full moon and in early morning after the full moon.) All students stand where the moon is visible.  Instruct students to hold the sphere in front of the moon so that the sun illuminates it.  The phase of the sphere is the same as the phase of the moon!

Why is this?

Just like the hand-held sphere, the moon is a sphere illuminated by the sun.  The sun is so far away, and the moon so close by comparison, that the angle of the sun to the moon is the same as the angle of the sun to the sphere.  (The sun is so big that its diameter is greater than the distance from earth to the moon.)

The phase is only the same when the sphere is right in front of the moon, along the line from the moon to the observer’s eye.  Whenever the sphere is in the sun, half its surface is illuminated by sunlight.  But we only see the part of the illuminated surface that is in front of our eyes.  We see the full disk of the sphere illuminated only when the sun is behind us.  We see half the disk when the sun is 90° from the line of sight to the sphere, and we see only a sliver (crescent) of the sphere when the sphere is back-lit by the sun.  It is the same with the moon.  We see the full round disk of the moon (full moon) when the moon is on the opposite side of the earth from the sun, and so on.

This would not work if the moon were not spherical.  You can test other round shapes and see that only a sphere gives the same light-and-shadow pattern as the moon.  So, you now know and have demonstrated that the moon is a round sky rock illuminated by the sun.

## 2. Kinesthetic Astronomy (Extension: phases of the moon and eclipses)

### Topic

Astronomy, orbit and rotation of the earth, seasonal changes in the position of the sun and day length, orbit of the moon, effects of sunlight on the climate and the appearance of the moon and stars

### Supplies

An exposed light or any object to represent the sun; room for students to circle around the “sun”; tags or marks on the different walls to represent stars; a globe and a darkened room is helpful as well if the “sun” is a light.  To extend the lesson to understand phases of the moon and eclipses, you really want a dark room and a central light source, and you will also want each student to hold a small sphere to represent to moon.  It’s best if the “moon” is mounted on a stick to avoid shadows from the hand holding it.  It’s also nice if the “moon” held at arm’s length is at least as big as the “sun” appears from the viewing distance.

### Procedure

Decide, either on your own or in consultation with the students, if “north” should be up or down.  Maps and globes are usually made so that north is up, but that is because their customers are usually north of the equator.  If north is up, all circling will be counterclockwise as viewed from above.  If south is up, all circling will be clockwise.

Place the object representing the sun at eye level in the center and stand the students in a ring around it.  The students each represent the earth in orbit around the sun.  (They will probably stand facing the “sun”, but do not require them to do that.)

Tell the students that they each represent the earth and that the central object (light) is the sun.  Direct them to turn around in place 360°, right side leading (counterclockwise) if north is up and left side leading (clockwise) if south is up.  Point out that as they turned, the “sun” “set” once and “rose” once; when they could see the “sun” was their “day” and when their backs were to the sun was their “night.” So one complete rotation is one “day”.  When they are facing the sun is their “noon.” Also point out that “noon” is a different direction for each of them.

But it’s not that simple.  The axis of the earth’s spin isn’t exactly perpendicular to its orbit around the sun.  That means that the students shouldn’t be standing straight up.  They have to tilt a little: 23.5° to be just like the earth, but that’s a bit much to try if you don’t want to fall over!  And the direction of the axis doesn’t change as the earth orbits the sun, so everyone in the room needs to tilt in exactly the same direction.  On one side of the circle, students lean their heads toward the sun; on the opposite side, they lean their heads away from it.  The students in between lean more or less to the side.  And as they turn, they must keep the same direction of tilt.  So, for instance, the students with their heads into the circle keep their heads into the circle as they rotate.  Have the students practice this a few times.

They should see that the sun probably doesn’t pass right in front of their faces as they rotate: if they are tilting away from the sun, they have to look down to see the sun at “noon;” if they are tilting toward it, they have to look up.  For those tilting in, their heads are in summer and their feet are in winter; for those tilting out, their heads are in winter and their feet are in summer.  Those right in between are in spring or autumn, depending on which way they are going.  That’s what comes next.

Of course, the earth doesn’t just spin on its axis; it also circles around (orbits) the sun.  So we need to do that too.  The yearly motion is in the same direction as the daily spin.  So, as students turn for a “day,” they also advance a little bit around the circle around the sun.  (If north is up, they move to the direction that is forward when the sun is on their left; if south is up, they move in the opposite direction: forward when the sun is on their right.) When they do this, they have to spin on their axes a little bit more than 360° from one “noon” to the next.  As they do this, which season are they moving into?  Which season are they moving out of?

If you have a globe, place or hold it at the same level as the light.  If the globe is in a stand, its axis is 23.5° from vertical.  If north is up, hold or set the globe so that its stand is level; if south is up, hold the globe upside down.  You can see where the sun shines on the earth.  Identify the part of the earth where it is daytime, where it is night, and where the sun is rising and setting.  Spin the globe slowly if it is in a stand.  (Spin it so that west follows east; counterclockwise looking onto its north pole.) Point out that as the earth spins, some of the earth moves into day, some moves into night, and that at all times some place experiences sunrise and sunset.  As the globe orbits the “sun”, the solstices are when the subsolar point is as far north or south as it gets; the latitudes of the solstice subsolar points are the tropics.  At these orbital points, note that the arctic circle experiences endless daylight and the Antarctic circle experiences endless night, or vice versa.  The equinoxes are the points in between, when the subsolar point lies on the equator.  At these orbital points, the whole earth experiences 12 hours of daylight and 12 hours of night.

If the students have “moons”, they can model them orbiting their heads, again in the same direction as their spin and their orbit around the sun.  The phase of the moon depends on the sun–moon–earth angle.  The plane of the moon’s orbit around the earth is angled a bit from the plane of the earth’s orbit around the sun, so most of the time the sun, earth, and moon don’t line up perfectly during the new moon and full moon.  When they do, however, eclipses can occur: solar eclipses during new moon and lunar eclipses during full moon.  Can students explain why lunar eclipses can be seen by half the earth at a time, while solar eclipses can only be seen by a lucky few?

## 3. Rock candy

### Topic

Solutions, solubility. supersaturated solutions, purification by recrystallization

### Supplies

Heat source (e.g. hot plate, mbaula, stove); pot for heating syrup; holders to safely handle the hot pot; stirrer; potable water; sugar; clean string; clean, heatproof cups (one for each student group); stick or rigid bar to place across the top of each cup; paper, plastic wrap, plastic bags, or foil to cover the tops of the cups

### Procedure

Cut the string into segments about 20 cm long.  Make one segment for each student group.  It is best, but not required, for the string to be pre-treated to contain seed crystals.  To do this, make a small amount of sugar syrup as described below, completely soak the string segments in it, and allow them to dry on a plate (covered to exclude vermin) for about two days.  You may sprinkle the strings with sugar to induce crystallization.

For the syrup: make about ½ cup (70 mL) of syrup for each student group.  Place a volume of granulated sugar equal to the volume of syrup you want into the cooking pot.  Add half that volume of potable water to the pot.  Heat the pot until all the sugar dissolves, stirring constantly.  If sugar crystals cling to the sides of the pot, cover the pot while heating without stirring, checking and stirring occasionally.  Remove the pot from the heat when all the sugar is dissolved.  Allow the syrup to cool until it is cool enough to pour into the cups. Pour equal amounts into the cups, and allow the syrup in the cups to cool.  When the syrup has cooled to room temperature, place a stick across each cup, hang a string over each stick so that it hangs into the syrup, and cover each cup to keep out pests (cockroaches love syrup).

Check the cups daily.  Sugar crystals should grow on the string (and on the sides of the cup) after a few days.  The crystals should be lighter in color than the sugar used, demonstrating the purification technique of recrystallization.  The sugar is purified in the crystals, while the impurities are concentrated in the remaining syrup.

## 4. Modeling Genetic Inheritance

### Topic

Mendelian inheritance, genetics, meiosis, expression of traits

### Supplies

Model of mother and father reebop; display of chromosomes of mother and father (heterozygous for all alleles); large genotype → phenotype chart (on flipchart paper, tagboard, or maize sack); envelopes or jumbos for “egg” and “sperm” chromosomes for each student group; colored markers for drawing eyes and noses; labeled baskets or other containers for body parts. length soft wires for tails, toothpicks, painted wooden matches for legs, nails for antennae.

Body part Original material TALULAR material
Antenna Nail or pin Nail or pin
Body segment, head Large marshmallow Small Irish potato
Tail Pipe cleaner Soft wire
Nose Thumb tack Ink spot
Leg Push pin wooden match
Eye Thumb tack Ink spot
Hump Miniature marshmallow Flower seed head
Connector Toothpick Toothpick

### Procedure

Make “egg” and “sperm” genomes by cutting two strips of paper of each length for chromosomes 1–7 for each student group.  For each chromosome, flip a coin to determine which allele is carried, for instance, heads for “A” and tails for “a”.  Write the allele on the chromosome.  Randomly place one chromosome of each length into the “egg” envelope or jumbo and the rest into the “sperm” jumbo.  Place an “X” chromosome into the “egg” jumbo and flip a coin to determine if an “X” or “Y” chromosome goes into the “sperm” jumbo.

Display the mother reebop with her genotype and the father reebop with his genotype in the classroom.  Display the genotype → phenotype chart as well.  Sort the materials for making the reebop bodies by body part and arrange them in the classroom by part so that students can easily select the parts they need.  Set aside a place for the reebop nursery if you don’t want students to keep their reebops with them at the conclusion of the activity.

#### Learner

Open the “egg” and “sperm” envelopes and take out the chromosomes.  Match each egg chromosome to the sperm chromosome of the same length.  Arrange the chromosomes into the karyotype of your baby reebop.  Write down the letters you have obtained for the genome of your baby reebop. For example, if you have one chromosome with the letter A and another chromosome with the letter a, your genotype is Aa.

Use the genotype → phenotype chart to decide what the characteristics (phenotype) of your baby reebop will be based on your genotype description. For example, if is genotype is BB, it will have 3 body segments.

Collect all the materials you need for your baby reebop body parts. For example, for 3 body segments you need 3 Irish potatoes.

Build your baby reebop with the characteristics that its genes determine.  Join the segments with toothpicks.

Put your baby in the reebop nursery with the other newborns!

#### Questions to consider

• Does your baby reebop look exactly like its mother or its father?
• Are all traits inherited in the same way?  Does each trait have a dominant and a recessive version?
• Did all of your baby reebop’s genes come from its mother or its father?

### genotype → phenotype chart

Characteristic Genotype/ phenotype code
body segments BB = 3 body segments Bb = 3 body segments bb = 2 body segments
tail TT = curly tail Tt = curly tail tt = straight tail
nose NN = red nose Nn = orange nose nn = yellow nose
legs LL = blue legs Ll = blue legs ll = red legs
sex XX = female XY = male
eyes EE = 2 eyes Ee = 2 eyes ee = one eye
humps HH = 1 hump Hh = 1 hump hh = 3 humps

## 5. Waves in a Rubber Band

### Topic

The nature of waves; wavelength, frequency and amplitude; wave speed; polarization; superposition; reflection; quantization of energy levels

### Supplies

rubber cords, both the stretchy and the taught kind, one per student group, two stretchy cords if they are shorter than 2m or if you just want more fun; rigid rod, such as a wooden stick; length of heavy thread, string, or light cord; tie-down location for each student group (window bars work well).  For extension: Slinky

### Procedure

Tie one end of a stretchy cord and hold the other end under tension in your hand.  Strike the cord near your hand.  Or, “pluck” the spring by holding the end fixed and lifting a nearby point, ( ) then letting go.  What happens?

Strike or pluck the cord in different directions: from above, from beneath, from the left, and from the right.  What is the wave pulse like in these situations?

What happens to a pulse when it reaches the fixed end?

Stand farther away so that there is more tension on the spring.  Again, strike or pluck the cord near your hand.  Is there any difference from the previous times?

Move the free end up and down, slowly at first, and then faster.  With a little practice, you should be able to produce patterns known as standing waves.  Although the cord moves up and down, the waves stay in place.  The places where the cord does not move are called nodes; the places where the cord stays in place are antinodes.  You should be able to make different patterns.  How many antinodes can you make in a standing wave?

How are the frequencies of the standing waves related to their wavelengths?  Is there a way you can measure the frequencies and wavelengths?

Remain where you are and change the tension in the spring or cord by feeding or taking up some slack.  Create the same standing wave patterns as before.  How do the frequencies compare to the frequencies of the same patterns under different tension?

Can you also make standing waves by moving the spring side-to-side rather than up and down?

Untie the cord from the tie-down and tie it to a length of light string.  Tie the other end of the string to the tie-down.  When you make a wave pulse in the cord, how does the pulse behave when it reaches the string?

Repeat with a cord that does not stretch so much under tension.  How do waves behave in it?

### Extension

With a Slinky, it is possible to show longitudinal as well as transverse waves.  A slinky sags a lot under gravity, so it is necessary to rest it on a level surface, such as a table or the floor.  Be aware that friction absorbs a lot of the energy in that circumstance.

#### Questions to consider

• What determines how fast a wave pulse travels along the cord?
• What are standing waves?
• Under what conditions can you make standing waves?
• How are standing waves related to traveling waves?
• What is the relationship between frequency and wavelength of a standing wave?
• Is the concept of wave speed meaningful for standing waves?
• Only certain specific standing waves can be sustained in the cord.  What characteristics do the “allowed” waves share?

## 6. Cartesian Divers

### Topic

Pressure; Pascal’s principle; compressibilities of liquids and gases; buoyancy

### Supplies

For each student group: plastic soft-drink bottle with a tight recloseable lid, preferably 2-L; glass vial or test tube, narrow enough to put into the soft drink bottle, with equally-spaced lines written in permanent marker; bucket or sink; pitcher of water; posted example of data table

### Procedure

1. On your own paper, draw a data table like the one displayed.
2. If your test tube or vial is not already marked, draw lines at 1-cm intervals from the bottom (closed end) to the top (open end).  Holding the bottom up, number the marks, starting with “1” at the closed end and increasing toward the open end.
3. Fill a plastic bottle with water.  Also fill the test tube or vial with water.  Quickly invert the test tube into the mouth of the plastic bottle so that a small bubble of air is trapped in the test tube.  If you wish, allow more air into the test tube by briefly lifting it above the surface of the water and then lowering it back down.  The object is to have enough air in the tube for it to float.  Let go of the test tube.  If it sinks, take it out of the bottle (as tidily as possible) and repeat the process, this time allowing a larger air bubble into the test tube.
4. With the test tube floating in the plastic bottle, top off the bottle so that it is as full of water as possible.  Cap the bottle tightly.
5. Squeeze the bottle in your hands.  If you squeeze hard enough, you should be able to make the test tube sink.  If you cannot, remove the test tube, refill it with water, and again float it in the bottle using a smaller air bubble.
6. Stop squeezing the bottle.  Record the “air level” in the test tube in the “no squeezing” column of the table.
7. Squeeze the bottle so that the test tube “diver” is just at neutral buoyancy: a little more squeezing makes it sink, and a little less makes it rise.  Record the test tube’s “air level” in the “neutral buoyancy” column of the data table.
8. Now squeeze the bottle as hard as you can.  Record the “air level” in the “maximum squeezing” column of the data table.
9. Remove the test tube form the bottle and re-float it as before, using different amounts of air in the test tube.  (That means different air levels in the “no squeezing” column.) Record the “neutral buoyancy” and “maximum squeezing” air levels in the table as before.

#### Questions to consider

1. What happens to the volume of the air bubble when you squeeze the bottle?  What happens when you squeeze harder?
2. How are the “neutral buoyancy” air levels related to each other?
3. What changes when you squeeze a capped 2-L bottle?
4. What happens to the amount of air in the test tube when you squeeze the bottle?
5. What happens to the amount of water in the test tube when you squeeze the bottle?
6. Why does squeezing the bottle make the test tube “diver” sink?

### Data table: test tube “air levels”

no squeezing   neutral buoyancy   maximum squeezing

## 7. Newton’s third law

### Topic

Forces are always interactions between objects: every force on an object is exerted by another object, and in turn the other object receives a force equal in strength in the opposite direction.

### Supplies

Rope, rolling platform such as a skateboard or other low-friction surface; tie-down location

Optional: football

### Procedure

Advanced students: Ask students: What is Newton’s third law?  (Expected answer: for every action there is an equal and opposite reaction.) Follow up with: What is a force?  (Expected answer: a force is a push or pull.  Ideal answer: a force is an influence that changes an object’s velocity.)

Beginning students: Ask students: When you kick a football, your foot feels the ball.  [Demonstrate kicking a football.] Which force is greater: the force your foot exerts on the ball, or the force the ball exerts back on your foot?  (Expected answer: the force exerted on the ball is greater.  Correct answer: the two forces are exactly the same strength.) How about when you kick a wall?  [Demonstrate kicking a wall.] How would you compare the forces on the wall and on your foot?

Explain Newton’s third law: When any first object exerts a force on a second object, the second object exerts an opposite force on the first object.  By “opposite force”, I mean that it has the same strength but is in the opposite direction, and along the same line of interaction.

This has some interesting consequences:

• All forces are interactions between objects: there can be no force on an isolated object
• Every force exerted on an object must be exerted by another object

So why, when you kick a football, the football speeds away while you stay in place (or keep running)?  How does the ball’s mass compare to your mass?  How about your mass and the mass of the wall you kick?

### Demonstration of the force in the opposite direction

Tie a rope to a light object and pull on the rope, pulling the object to you.  Clearly, you apply a force toward you on the object.

Then tie the rope to a fixed object and stand on the skateboard.  Pull on the rope as before.  This time, it is you that moves—toward the fixed object, opposite the direction that you are pulling!  The fixed object is pulling you toward it, just as you pulled the lighter object toward you.

### Stubborn ox question:

You hitch your ox to a ngolo and tell it to pull.  To your surprise, the ox protests that there is no point in pulling, because the ngolo will pull backward on him just as hard as he pulls forward on the ngolo.  What is wrong with his thinking?

### Wrap-up question:

Which force is stronger: the pull of the earth’s gravity on the Moon, or the pull of the Moon’s gravity on the earth?  (Correct answer: they are equal in strength although in opposite directions, each toward the other body.)

## 8. Challenge: Drop a box of matches so that it does not bounce

### Topic

Collisions; elastic and inelastic collisions; impulse, momentum, kinetic energy, and work

### Supplies

Box of matches per group

### Procedure

Distribute a box of matches to each group.  Challenge them: can you drop the box on its end from 1 foot (30 cm) above the desk so that it does not fall over?

They will try and try, and some may even manage to luckily make the box bounce and land back upright.  But overall it cannot be done consistently.  Unless, that is, some enterprising students find the trick.

After a while, show them the trick.  (The trick only sometimes works for me, but it gives a much better chance than without.)  Slide open the matchbox so that the sleeve lands first and the drawer closes by momentum.  The friction of the drawer sliding shut absorbs the kinetic energy of the falling matchbox, making the collision more inelastic (the matchbox stops dead rather than bounces).  The kinetic energy of the falling box is absorbed gradually, over a relatively long time and distance, rather than quickly.  I have found that it helps to put more mass in the matchbox, using a small coin, washer, nail, or pebble.

#### Questions to consider

• What are some applications of this principle?  (Cushioning the fall of a jumper; breaking rocks)
• What other objects behave in this way?  (Landing with bent knees when jumping; crumple zones in automobiles; catching a thrown object.)
• When might we want a collision to be inelastic?  (Automobile collisions, catching a ball)
• When might we want a collision to be elastic?  (Driving nails, kicking a football)
• What other conditions determine how much colliding objects rebound?

#### The science details

The greater the matchbox’s speed v or the greater its mass m, the more difficult it is to stop.  What do we mean by “difficult”?  Its momentum mv and its kinetic energy ½ mv2 must be reduced to zero.  Both momentum and kinetic energy become greater as mass and speed become greater.  To change momentum requires an impulse Ft, where F is an applied net force and t is the time over which it is applied.  To change kinetic energy requires work Fd, where F is the applied net force and d is the distance over which it is applied.  Both of these require a force, but the needed force is smaller as the time t and distance d are longer.  The acceleration, or rate at which the velocity changes, is F/m.  When the matchbox is open, only the sleeve stops immediately upon impact.  The majority of the mass, in the drawer, slows and stops over a longer time and distance, with less force and acceleration.

## 9. Centering an Object

### Topic

Physics, friction, torque, balance, center of mass

### Supplies

One extended rigid object per group.  Each object must be about 1 to 1.5 m long, light enough to rest on two fingers, and smooth enough to slide along a finger without catching.  Suitable objects include thin boards, meter sticks, any implement with a long handle or the handle itself (mop or mop handle, axe, hoe).

### Procedure

Hold your forearms parallel to the floor with your hands extended, palms facing inward.  Your hands should be at least shoulder width apart.  Rest the object on your index fingers.  Slowly bring your hands together, keeping them level, until they meet.  What happens to the object?

If you brought your hands together slowly and steadily enough, it should still be balanced on your fingers!

Now begins the real work of this activity: finding out why.  What sort of “feedback” kept the object balanced on your hands throughout the exercise?  Did it depend on any conscious action from you?  As a group and then as a class, make a thorough explanation of why the object remains balanced and centered.  Carry out additional experiments if more questions arise.

#### The science details

The closer a support finger is to the object’s center of bass, the more of the object’s weight it supports.  The greater the weight on a finger, the greater the force of friction to keep the object from slipping.  So the object will stick to the support finger that is closest to its center of mass.  As the fingers move together, the object’s center of mass stays between them.

## 10. Surface tension and detergents

### Topic

Surface tension; solubility

### Supplies

Water, the purer the better; small basins, one for each student group; liquid detergent; soap; hydrophobic powder (cinnamon, ground pepper, lycopodium or other pollen); small paper clip, needle, or fine wire for each student group; forceps if available, forks if not; waxed dental floss

### Procedure

If time permits, try using soap in all of these procedures where detergent is called for.

#### Floating paper clip

1. Place clean water in a clean basin to about 5 cm depth.  Make sure that there is no crud in the beaker, and no oily scum on the water.  Everything should be as clean as possible.
2. Make sure that your paper clip, needle, or wire is clean and dry.  Gently hold it with forceps, scissors, or a fork and lower it to the surface of the water.  Release it so that it floats on the surface of the water.  If it does not float, recover the paper clip, carefully clean and dry it, and try again.
3. Look at the floating paper clip, both from above and with your eyes level with it.  What do you see?
4. Now place a tiny amount of detergent on the end of a toothpick and touch it to the surface of the water some distance away from the paper clip.  What happens?
5. Recover the paper clip, rinse it off, and carefully dry it.  Try to make it float on the surface of the water again, as you did before.  What happens?
6. Can you make another new, dry paper clip float on the surface?  What must you do to accomplish this?
7. When you are finished, wash the basin well with detergent and water, and place it in the drainer to dry.  Similarly wash the paper clips, rinse well, and dry.

#### Floss loop

1. Fill the bowl with clean water.  Float a loop of waxed dental floss on the surface of the water.  The loop should be open, but oblong rather than round.  () The entire loop should lie on the surface of the water.
2. Touch a drop of detergent to the water surface inside the loop.  What happens?
3. Dump out the water and rinse the bowl and the floss loop with clean water.  Refill the bowl with clean water.  Float the floss loop on the surface, this time making it as round as possible.
4. Touch a drop of detergent on the end of a rod to the water surface outside the loop.  What happens?
5. Clean up when you are done.

#### Ground pepper

1. Fill the bowl with clean water.  Sprinkle ground pepper (cinnamon or lycopodium powder will also work) onto the surface of the water. The pepper should be evenly distributed over the entire surface of the water.
2. Touch a drop of detergent to the water surface near the center of the bowl.  What happens?
3. Clean up when you are done.

#### Questions to consider

• What did the surface of the water look like when the paper clip was floating on it?
• What causes a buoyancy force on an object submerged in a fluid?
• Can the (upward) buoyancy force on the paper clip be as great as the paper clip’s (downward) weight?
• When the paper clip was floating on the surface of the water, what force kept it from sinking?
• What did adding a small amount of detergent do to the surface of the water?
• How did detergent keep the paper clip from floating on water?
• What forces acted on the dental floss on the water surface?
• What made the dental floss and pepper move on the surface of the water?

#### The science details

Water molecules are strongly attracted to each other.  When water contacts another material to which it is not so strongly attracted, such as air or oil, it forms a layer at the surface that is under tension, like a sheet of stretched rubber.  This layer is strong enough to support light objects that are heavier than water, such as a paper clip or an insect.

Soaps and detergents are surfactants (short for surface active agents) stabilize the surface, so that it is not under such tension.  A small amount of detergent added to the surface of water is pulled to a thin film by the retreating surface tension.  You can see this by the movement of items floating on the water surface.

## 11. Sensitive Skin

### Topic

Sensitivity, sensation, tactile illusion

### Supplies

Length of soft wire, ruler, blindfold (optional)

### Procedure

For best effect, this activity should not be done individually.  It should at least be done in pairs.

Distribute the supplies to each group.

Open by reminding them that when something touches your skin, you can feel it.  And you also can feel what shape it is.  In this activity, they will find out how accurate that feeling is.

One person in the group will have his or her eyes closed while another touches his or her skin with one end of the wire, or with two ends at the same time.  The person being touched must decide if it is one or two.  The person doing the touching bends the wire into a “U” or “V” shape, varying the distance between the two ends.  Try to find how far apart the ends can be for the person being touched to still think that it is only one touch.

Find this critical separation for different places on the body.  Are they the same for the different members of your group?

Think of reasons why our skin has these different sensitivities in different locations.

### The science details

The sense of touch does not have the same sensitivity and resolution everywhere on the body.  In certain parts of the body, such as fingertips and lips, the sense of touch is used to give us detailed information about what we are touching.  In other parts, the sense of touch is just used to let us know where things are so that we don’t injure ourselves.

## 12. Isolating DNA

(I have not checked this activity with African materials.  I have done it in the United States using raw wheat germ.  I do not know if raw wheat germ is readily available in Africa, or if maize germ can be obtained in a form that is suitable for this activity.)

### Topic

Biochemistry, DNA, purification of biological materials, properties of biopolymers

### Supplies

Raw wheat germ; methylated spirit; detergent; hot water; 50-mL jar; paper towel; Pasteur pipette or dropper (can be shared between student groups); inert stirrer, such as a glass rod

### Procedure

I need to check this out myself first for the DNA source I’m thinking about; it may be better as a longer lesson (though 40 minutes isn’t much longer than 30 minutes).

1. Place 1 gram or 1 teaspoon of raw wheat germ in a 50 ml test tube, beaker or jar.
2. Add 20 ml or 1 tablespoon of hot (50–60 °C) tap water and mix constantly for 3 minutes.
3. Add 1 ml or a scant 1/4 teaspoon of detergent and mix gently every minute for 5 minutes. Try not to create foam.
4. Use an eyedropper, pipette, or piece of paper towel to remove any foam from the top of the solution.
5. Pour the water/detergent solution into a clean test tube, leaving behind the wheat germ.  Tilt the test tube, beaker or jar at an angle. SLOWLY pour 14 ml or 1 tablespoon of methylated spirit down the side so that it forms a layer on top of the water/wheat germ/detergent solution. Do not mix the two layers together. DNA precipitates at the water-alcohol interface (the boundary between the water and the alcohol). Therefore, it is crucial to pour the spirit very slowly so that it forms a layer on top of the water solution. If the spirit mixes with the water, it will become too dilute and the DNA will not precipitate.
6. Let the test tube, beaker or jar sit for a few minutes. White, stringy, filmy DNA will begin to appear where the water and alcohol meet. You will usually see DNA precipitating from the solution at the water-alcohol interface as soon as you pour in the alcohol. If you let the preparation sit for 15 minutes or so, the DNA will float to the top of the alcohol.
7. You can usually get more DNA to precipitate from the solution by using one of the DNA-collecting tools (such as a glass or paper clip hook) to gently lift the water solution up into the alcohol. This allows more DNA to come in contact with the alcohol and precipitate.
8. Use a glass or paper clip hook or a wooden stick to collect the DNA.
9. If you want to keep the DNA, store it in 50–70% alcohol in a sealed tube or air dry it on paper towels or filter paper.