The simple things are often the best.
From my dim and distant past...

1.)
Take a matchbox. Remove phosphorous(?) tips from match heads (either simply cut them off or scrape the redheads off the stick). EDIT: Maffer's idea of pounding them would also work - just make sure you don't pound them with anything resembling the striking strip!
EDIT: EDIT: There's nothing I like more than scraping a redhead off my stick...
Remove striking strips from box, fashion into 2 "U" shapes.
Place matchhead material in 1 u shape, cover over with other strip to make a nice neat cube.
Seal and tape cube up tightly with sticky-tape. The more the better. You should have started with a cube about 100mm, you should finish with a golfball sized stickytape ball.

If you have a Fast ball pitcher in your midst ask them to ditch it at a sturdy nearby wall. Chances are not much will happen.
Get a big heavy rock. Place matchhead bomb on ground and drop rock on it from a decent height - I suggest about 3m.
Rock will change into shrapnel very quickly. Kinetic energy will be transformed into sound waves. Anything nearby if not broken will be converted into shit-scared instantly. Car alarms may be triggered.
Topics covered - geometry, physics & some chemistry.

2.)
Acquire some dot 4 brake fluid and some granulated pool chlorine.
Place brake fluid in a sturdy (but inexpensive) container - preferably thick glass as it will look spectacular when it shatters.
Quickly pour about a cupful of chlorine into the brake fluid and RUN THE FUCK AWAY whilst trying to look back to observe your handywork. Note chemical energy rapidly being converted into sound and light.
Gasp appreciatively.
Topics covered - chemistry, aerobic exercise.

Note: no precise measurements or suggestions for safety gear have been provided as if you are either young or stupid enough to attempt either of these experiments then you deserve whatever comes your way.
Good luck intrepid pyros.
(Misery McUglywife an attention seeking sociopathic fuckstain., Sat 11 Aug 2012, 0:54, 3 replies)

Now tell them about the other experiment where you decided to see what would happen if you, a married man with children, put a profile up on OKCupid.

( Amorous Badger NAKED BEA ARTHUR PHOTOS 4U, Sat 11 Aug 2012, 10:20, closed)

You've been talking to Rory haven't you?

(Misery McUglywife an attention seeking sociopathic fuckstain., Sat 11 Aug 2012, 12:59, closed)

You and Rory had an illicit encounter through OKCupid?
This just keeps getting juicier.
( BraynDedd Natasha Kaplinsky's labia majora, Sat 11 Aug 2012, 13:35, closed)