This is a fun way to demonstrate the surface tension of water.
2) Fill something with water.
3) Gently place rocket in water.
4) Drop liquid soap into circular opening.
5) Blast Off!
6) Dry off rocket, get new water, and start again!
There are two versions, the only difference is V2 is about half as thick as V1. Both seem to work about the same.
Check out the video of it in action: http://youtu.be/00hEP6iZUvs