Sodium hypo-chlorate + sugar bombs are one of the fastest ways to make explosions that i know of.

INSTRUCTIONS

MATERIALS:
1 part Sodium hypo-chlorate
1 part Sugar
3 part room-temp Water (20°C)
600mL plastic bottle (coke brand bottles are the best)

EQUIPMENT:
5mL (one teaspoon) measure

METHOD:

1. First you need to measure out the sodium hypo-chlorate. I suggest using 5mL for a part is you want a slow build-up of pressure inside the coke bottle before exploding, or 10mL for a instant bang. Anyway, pour the measure into the coke bottle.

2. Now pour in the measure of sugar, and give the bottle a shake.

3. Finally pour measure of water into bottle, screw cap on hard, shake for about 10 sec and then put a fair distance between you and the bottle. Depending on the mixture and temperature, the sodium hypo-chlorate and sugar should start to react between 30sec and 60sec producing large amounts of gas. If you used 5mL for 1 part, the bottle should look like its getting fatter and fatter and then finally explode. If you used 10mL for a part, as soon as the reaction starts it will explode. This explosion can take out enclosed things, such as letter boxes (I never told you that :-).