[punatic]

Using different amounts of sugar, show that the size of the balloon is proportional to the amount of food the yeast have. Explain that yeast eat food and expel CO2, just like people do.

I think people expel methane, at least from the anterior direction anyway.

[Slowbrew]

Here's something I remember doing as a kid...

Raw materials required:

Matches or lighter
4 empty, opened cans of same size (average size, like the normal canned corn can)
Wire Hanger
Duct Tape
rubbing alcohol
Tennis ball

The tennis ball should fit moderately snugly in one of the cans.

First take three of the cans...drill a largish hole (maybe 1 1/2"?) in the base of the can.  This will leave a baffle ring around the edges, this is fine.  Take the fourth can, and drill a small hole (maybe 1/8") in the side of the can near the base.  Then duct tape them tightly together, with the fourth can as the base, the others on top.

Bend the wire hanger so it constitutes a nice stand...mold the main hanger to prop the "cannon" up, and bend the hook straight so it sticks in the ground steadily.

Load the tennis ball into the end of the cannon, it should fit down into that first can.  Using an eyedropper, shoot a couple squirts (I really don't remember how much) of rubbing alcohol into the hole in the base.  Roll it around for 30 seconds or so, so it covers surfaces on the inside.  It will start evaporating within the cannon.  With the cannon facing a safe direction in the stand, touch a lighted match or lighter to the hole in the base.  

Boom!  I had a lot of fun with this when I was younger.

Hah! Cool! But remember, You'll put an eye out! How's that larger hole figure in to everything? Can this be used to annoy squirrels?

Look up Potato Cannon or Spud Gun.

[loopy]

Tonight my daughter won third place in the science fair this year (4th grade). And she got a "Judges Choice" ribbon too!  Tomorrow she gets to pick dinner. 

[tschmidlin]

Awesome, congratulations to her!

[punatic]

Tonight my daughter won third place in the science fair this year (4th grade). And she got a "Judges Choice" ribbon too!  Tomorrow she gets to pick dinner. 

Stalagmite Stew?

Congratulations!  Cool project!!

[capozzoli]

Here is a simple one that involves our sensory perception.

Take a cup of water and put ice in it. Fill another cup with hot water (not so hot that you cant put your finger in it) Then take a third cup and fill it with room temp water.

Put a finger from your right hand in the ice water and a finger from your other hand in the hot water, wait several seconds and then put both fingers into the room temp water.

In this same glass to one finger the water will feel cold, the other the water will feel warm.

[punatic]

Some people pay a lot for that trick...

[oscarvan]

I am proud of all of you. Good parents while nowhere near acting your age.... I am in awe.

[weazletoe]

Potato cannon.
PVC tube - propane grill spark igniter - hair spray propellant and potato projectile.
Standard fare at Troop 65 campouts.

+1000. 

That's not a knife!! THIS< Is KNIFE