Do a proportional map of the solar system. Start with the sun the size of a golf ball. Calculate the size of the earth and the other planets as a proportion and make them out of paper mache. (The earth will be like the size of a BB.)

Then calculate the distance from the sun to the planets, pace it off on the school grounds, and place the model planets in their proper places.

This really shows kids how much space there is between the planets, how far apart the planets are and how tiny the Earth is in the solar system.

Check out the Disgusting Science kit. My daughter has this, and it has everything you need to do some very cool (and rather gross) science experiments. One of the things my daughter did was use the petri dishes to grow cultures of bacteria and mold from various parts of our house. She collected samples from the bathroom sink drain, the underside of the toilet bowl rim, and the fishtank. I’m sure there are online resources you can use to help identify what you’ve grown, such as bacteria vs. mold. If you have a microscope, your child could study the bacteria and classify it by shape (circular, rod-shaped, etc.). We didn’t do all this, since we were only having fun we’re deranged and not working on a school project.

If all that is more than you want to know, you can simply compare bacterial growth rates in different environments. You’d need one petri dish as a control, but use the same sample source for all of the other petri dishes. Put one in a warm, dark place, one in a cool place, one in the sunlight, etc. After a week, compare how the cultures grew in the different environments. Can you tell bacteriology was one of my favorite classes in college?

What’s great about the bacteria-growing is how many different comparisons you can make. You can sample your hands before and after washing them. You can take swabs of different raw meats. You can compare samples taken from different people’s mouths to swabs from pets’ mouths. Heck, you can even take samples from around the school if you really want to stir things up.

There are a lot of other things you can do with the kit, but the petri dishes were the most memorable for us.

One science project (one-time rather than over a period) that I’ve used is to have a group of kids (6th grade) act as neurons. You have several stand in a line ten feet apart from each other and pass a ball down the line, this represents the ‘jumping’ nerve impulse down the axon. When the ball arrives at the end, another student throws tennis balls at a board or net with holes in it, this represents the neurotransmitters binding to the receptors at the neuromuscular junction. Finally, when a certain threshold of tennis balls go into the hole (3–4), the muscle action is triggered as represented by some one shooting a ball at a basket (if in a gym) or throwing the ball to some one standing further away.

Have two groups do this, explaining the various aspects of the nerve that their actions represent. Have them go through it a few times to compete for speed (this usually gets their attention and cooperation). Then explain about how the myelin sheath on the axon allows for the fast jumping signal (them throwing the ball to each other) and how in people with myelin related diseases like multiple sclerosis (MS) this gets disrupted, slowing down the signal. Have one team, and then the other for fairness, be the demyelinated nerve and have to run the ball to each other instead of throwing it. The difference is dramatic and the message about myelin along with some of the facts about how nerve cells are set up and communicate is usually pretty easily grasped.

Build a perpetual motion machine.

Tye dye coffee filters
You will need:
-White coffee filters
-Different colored food coloring/or any type of dye. 
-Rubberbands
-A plate

Directions:
1.) Fold the coffee filter in half and then half again till it looks like a triangle.
2.) Tye it with rubberbands where you want the sections to be at.
3.) Dip it in the dyes and make sure you have the plate under it so it won’t stain everything else. 
4.) Don’t unfold it till it dries completely.

Or
Borax Crystals
You will need:
– Borax Powder
– Hot water
– Coffee stirring sticks
– Pipe cleaners
– Disposable cups
– Thread

Directions:
1.) Put in about a cup of hot water in one of those disposable cups.
2.) Add about ¼ cup of borax in there. 
3.) Mix it with a spoon till the water becomes clear again.
4.) Have the kids make any shape they want with the pipe cleaner. (make sure it will fit in the cup)
5.) Tye a piece of string to it and then tye it to the coffee stick.
6.) Lay the stick on top of the cup.
7.) Put it in the boraxed water and make sure the pipe cleaner doesn’t touch the bottom of it.
8.) If you need to make the string longer or shorter just cut it, the stick will keep it from falling down.
9.) Keep it in there for 24 hours, you can see the crystals start forming around the pipe cleaner.
10.) Drain the water and lift the stick and there you have your crystal.
Ps. This can also be a good idea to make christmas tree ornaments too.

Throw lots of sodium in some water. Everyone will be happy :P

Next time you fly take an empty closed plastic bottle with you. At cruising altitude open the bottle and listen to what happens. Close the bottle. After landing look at the shape of the bottle. Open it again and listen again.

Make oobleck – cornstarch and water.
http://www.youtube.com/watch?v=Yp1wUodQgqQ
It is messy but the kids love it.

I liked exhibits having to do with electricity or volcanoes and other geological phenomena.

Things have to go boom! or zzzpt! for me at the science fair. A diagram of the Krebs Cycle isn’t gonna do it.

Curse that evil Krebs cycle, the bane of biology students everywhere.

Write words for colors using different colors, I.e. “red” written in purple ink.

Have kids read them and tally how often someone reads “purple” when the word is “red” (in purple ink). Freaks people out how often one’s brain tricks yhim or her!