I wasn't really sure what to call this - an air powered pinwheel? Don't pinwheels go more horizontally? Maybe it's a propeller? Anyway, it's fun and it's science!
Anyway, you'll need a balloon, a bendy straw, a pencil, a pin, and tape.
Blow up the balloon a few times to stretch it out a bit.
Place the baloon over the end farthest away from the bend and attach with tape.
Bend the bendy part at a 90 degree angle
Next you'll need to help out by putting the needle through the straw into the eraser as far as you can push it in while leaving room for the straw to spin. You'll want to place the needle closer to the balloon end than the bent end to even out the weight a bit.
Blow up the balloon through the end of the straw.
Let her rip! The air being forced from the balloon through the straw will cause the straw to spin in circles like a propeller. It's hard to show this through photos but I'll give it a shot. Then I'll go and learn how to shoot and edit video ;)