Here's a fun easy-to-make gadget, an idea obviously stolen from somewhere.

Take a plastic 32 oz. wide-mouthed bottle--Snapple works well. Burn a tiny hole in the bottom using a hot paper clip.

Wrap the lip of a balloon around the mouth of the bottle. Blow up the balloon, but put your finger on the hole whenever you are not actually inflating the balloon. The kids can look directly into the inside of a fully inflated balloon.

(If you are in a particularly evil mood, give a student the same set-up except "forget" to put a hole in the bottle.)

This is a great opening exercise, gets the students thinking about air pressure and such, and I still get a bang out of looking into a wide open inflated balloon.

But the best part is watching my freshmen actually take notes when I announce, in my best stentorian voice:

Nature never sucks....long pause....nature only blows.

Steve Spangler has an excellent website with all kinds of balloon in a bottle ideas.
If I can fire enough neurons to remember to take the camera tomorrow, I'll post a pic.