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What fruit has the most seeds?
This is a question I got a few weeks before Halloween - great timing. I threw the question back at the kids and told them to pick a fruit and find out. We talked about different fruits - some have 1 seed, some have seeds on the outside, some on the inside.
The kids brought in answers for apples, peaches, bananas, kiwi fruit, and
pumpkins. Clearly the winner was the pumpkin. I brought in the seeds from
our own Halloween pumpkin, and used them to show them you can make quick
estimates of large numbers by dividing the heap into 2 as equally as
possible, then count only half and multiply the count by 2 to get the
total. Do it again dividing into 4, 8, 16. Each time it is faster to count,
but the resultant estimate of the total gets worse. This of course connects
to statistical sampling and margins of error.
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Does everything have a name?
This of course leads to the story of the eskimos having many words for snow and the like, and we have words for things we think are important. New words are made up, borrowed, stolen as needed. I asked them if they thought there was a word for the little plastic thingie on the end of my shoelaces, and clearly this little object was not important enough to get a name. The very next day however, one of my colleagues at the lab brought up this very same object during lunch conversation, as an example of an obscure word that nobody knew (it's called an aglet, according to him). Now is that a coincidence or what? 
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How do rockets fly?
This one was an opportunity to shoot some water rockets, and do a few simple experiments. Recall that I am working with 3rd-6th graders, so you can't do differential equations to solve for the rocket's acceleration, nor use trigonometry to do triangulation. Here is how the kids actually measured the maximum height of the rockets under various conditions.
Materials: you can go to any of the water rocket links below here, but this
is what I used:
How to:
This one is about conservation of momentum. Links:
Bring:
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Why is the sky blue?
Now this is a question that a lot of scientists would have trouble with answering off the top of their head. However, it just so happened that I had been working for years with an exotic material called aerogel. I was using aerogel for it's optical properties. These properties are dominated by Rayleigh scattering, which is the same reason why the sky is blue. I didn't answer the question there and then, but put together the following classroom activity for the next week:
Now you have to make the kids look at all the arrows, and imagine that they are standing there on the earth. If they look up in the direction of the sun, they see the red and yellow light coming at them from the direction of the sun (look at the colored arrows). When they look off in some other direction, the light that comes to them is mostly blue.
What happens at sunset and sunrise? If time allows, you can make the sun set and play the game again. Since the path of the slanted light is now longer, the yellows and even some reds scatter. This means that the sun looks redder because more of the yellows go missing, but also the sky is more rosy-colored. Does the sun look the same color on the earth as it would from space? Not really; the blues that get scattered out of the straight path when you look up through the atmosphere can reach your eye without scattering when you're in space. Therefore the sun should look a bit whiter in space.
What color would the sky be if the atmosphere were thicker, say almost as thick as the red rays? What color would the sun be? Why is the moon red during a lunar eclipse? look here. And finally they should be able to answer this one themselves: Why is the sky blue? This last time I added the water/milk/flashlight demonstration, since it is simple and does not take up much time. Here you take clear container of water (preferably something with flat sides, like a small aquarium; I had a rectangular clear plastic storage container from my kitchen), fill it with water, and shine a flashlight through it (try to get the brightest one you can find, maybe even a slide projector). None of the light beam scatters. Now add some drops of milk. This makes a Rayleigh-scattering medium, and what you see is that the light scattered out of the beam close to where the light enters the water is bluish, and the light scattered further down is more orange. Also, the lightbulb as seen through the water looks orange, the color of the setting sun. Also, my daughter had just bought an egg-shaped rock called a moonstone, about 1.5", and this is also a beautiful Rayleigh scatterer, showing the same blue and orange hues. Finally, I brought in some aerogel, the world's lightest solid. Here are some links about the blue sky:
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On the floor, mark off the atmosphere with masking tape. One strip, about
15', is the earth's surface, and it gets the paper bushes, or whatever you
have to indicate it is the ground, including perhaps a little person.
The first kid picks a random arrow from the bag. The arrow represents a
light ray of that color. He starts at the sun, and points the ray towards
the earth, that is, towards our model atmosphere. In space, the ray can fly
unobstructed towards the earth, so the arrow is moved in a straight line
until the tip touches the top of the atmosphere. Now the rules change: he
now has to pick a string from the bundle with the same color as the ray he
has. This is the distance that the ray can travel through the atmosphere
before being scattered. So you take the string, and lay it out straight in
front of the paper arrow, and in the same direction. The paper arrow now
moves forward along the string, until its tip touches the other end of the
string. Then you get the spinner, and twirl it to choose a random new
direction. Grab the string, and lay it out parallel this new direction,
starting at the current arrow tip. Move the arrow again to the end of the
string. This game of step-change direction-step-change direction continues
until the arrow crosses a boundary: either it
hits the ground or it goes out of the top of the atmosphere back into
space (ignore the bushes and clouds in all this). When this happens, leave
the arrow at the boundary, in the direction in which it crossed.
If you're out in space, what color is
the 'sky' if you look in a
direction away from the sun, and away from the earth? No light scatters in
empty space, therfore space is black.
