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.

Materials: you can go to any of the water rocket links below here, but this is what I used:
For the rockets:

• A collection of bottles. I had a liter bottle, a 2-liter bottle and a smaller water bottle. Make sure they all have the same size opening.
• A rubber #4 stopper with a center hole that fits the bottles. I got mine at the local hardware store. Also try stores that sell wine- and beer-making equipment. If you can only find stoppers without holes, don't let this hold you up; I was able to make a good enough hole with a small utility knife. Once, at a friend's house, even an ordinary cork and a drill gave usable results.
• A valve stem I got from an old bicycle tire. The corner garage can also sell you a valve stem for a few pennies. Take out the insides and trim the outside rubber off.
• Hose extension (optional but very convenient). You can stick the valve stem straight into the stopper, but then you get wet every time. At the hardware store, look for plastic hose and fittings to make a 6' or so extension.
• Bicycle pump. Any pump will do, but keep your eye open for one with pressure gauge attached.
• A launcher. I had a 1-foot piece of 4" plastic pipe, into which I cut a slot to allow for the 1/4" hose. I put this straight up on a board which had some screws sticking up in a circle, and a bit of duct tape does the rest. Bottles up to the 1-liter model fit nicely inside this contraption, bigger ones balance on top.

• Measuring cup, to measure the amounts of water used for propellant.
• Two angle measuring devices. Each one consisted of a piece of plywood a bit bigger than a 8.5x11 sheet of paper. On the back of this was screwed a piece of 2x2, such that the whole thing could be c-clamped to the top of a regular photography tripod. On the front side, a big nail sticks out of the center of the board. On this nail pivots a 1' piece of 3/4" plastic pipe, held in place by a cork stuck onto the protruding end of the nail. A thread with a washer for a weight is taped to the top edge of the board.
  To use the device, pull off the cork and slip off the plastic pipe, and put a clean sheet of paper over the nail and tape it to the board. Put the pipe and cork back. With the board clamped to the tripod, use the tripod's controls to align the sheet with vertical, using the thread and washer, which should be made to dangle along one edge of the paper.
• Blackboard.
• Tape measure and a meter stick.
• Clipboards and paper.
• Assistants. For your average bunch of third-graders, three adults on the field is a minimum.
• Water supply. I carry two 2.5-gallon water containers. This is plenty for an hour's worth of action.

How to:

1. The launcher is set up in the middle of the field with a launching team, and two observation teams are set up on either side of the field, in my case, each 30 paces away from the launcher. Each team has an adult nearby. The observation teams set up their tripods, mount clean sheets and align them with the vertical. Make sure the sheets face the same way (e.g both north). Since the measuring devices are symmetric, it is possible to get this wrong.
2. Do a test launch. Have them pick a bottle, measure out some amount of water, and launch it. The observation teams are supposed to follow the rocket up with their tubes, and leave the tube at the highest position. Then they trace a line along the side of the tube, and write by that line the launch number. (Adults make sure it is clear which number belongs to which line - the sheet can get crowded). In the meantime, the launch team keeps a table where they write the launch number, which bottle got used, how much water was used, (optionally the pressure if you have a gauge; The first time I had no gauge, but I made sure that I jammed the stopper into the bottle with the same effort every time).
3. Immediately the kids will come up with a plan for the next launch: more water, less water, different bottle etc. I steered the action into a series of launches with the same bottle and different amounts of water. Be sure to include the 'no water' and the 'full bottle' variants.
4. After you have a related set of about 5 measurements, gather at the blackboard. The observation teams' data sheets are taped to the bottom left and bottom right corners of the blackboard. Measure the distance on the field in steps (assuming 1 step=3'). In my case 60 steps between tripods, and divide the distance between the corners of the blackboard into as many marks.
   

   Click here for a closeup of what's written on the board.

5. Now you extend the lines from launch 1 from the papers sheets onto the blackboard to where they intersect, and use the scale that you drew on the bottom of the board to measure the altitude that was reached.

This one is about conservation of momentum.

Links:
There are lots of water-rocket web sites out there, but I'll send you to my friend Gordon McDonough's site, and he has all further links: Off you go to the

• Interplanetary Water Rocket Society.
• My own notes on water rockets,
• and Clifford's site

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:

Prepare:
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.
About 3' away from the ground is another strip of tape, marking the top of the atmosphere. The thickness of the atmosphere should be less than the length of the red strings, about the same as the average length of the yellow strings. Place the clouds (birds, airplane) in the atmosphere. The sun gets placed far away from all this, like across the room.

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.

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.

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:

• a good site on the blue sky
• Much about the atmosphere at Science Made Simple
• Ultra-condensed explanation
• Blue sky at the San Francisco Exploratorium
• A nice slideshow
• lots on the sky, atmosphere ect on Enchanted Learning