How do you build a 2 liter soda bottle rocket?

Step 1: Wear Eye-Protection

Why settle for one 2l bottle? Mythbusters took it one step further:
http://en.wikipedia.org/wiki/MythBusters_(season_3)#Bottle_Rocket_Blast-Off

you can use one of those massive jugs on the top of a water dispenser.
This guide works for 2L bottles too…

1. Fill half of the bottle with water.
2. Turn bottle upside down.
3. Cork / Bung it
4. Attach fins to the side of the bottle for direction on its journey through the atmosphere.
5. Stand bottle with it’s top facing the floor and place a VERY long tube into the cork.
6. Withdraw to a safe distance (25m maybe?)
7. Pump the tube full of air as to equalize the pressure so that the water within the bottle doesn’t escape, through the tube.
8. Continously pump through the tube, and eventually the cork will pop due to the enourmous amount of pressure built up insde the bottle.
9. Watch it fly.

This works amazingly well with the really big bottles because you can build up more pressure inside them before they pop.

good luck.

Mentos and a bottle of diet coke. : )

http://www.fluther.com/disc/12404/alright-how-do-you-make-a-simple-water-rocket/