Slashdot featured this cool site detailing the adventures of a "water rocket" enthusiast attempting to create a new power source for one pink Barbie-mobile.

The water rockets I remember were cheap plastic store-bought things with a tiny bycycle-pump to pressurize them. You filled the rocket, which was about 8" tall, about 1/4 full of water by turning it upside down under a spigot. Then you clipped it to the end of the pump with a kind of sliding lock. You then turned it right side up and pumped the hell out of it. When you just couldn't pump any more, you slid the lock back and FWOOSH (a technical term, you know), it would disappear in a cloud of water vapor, typically to crash down on a) your brother's head (the most desired outcome), b) your head (rare) c) a tree or d) the neighbors house (very common).

But those are literally toys compared to the stuff described in the article, and what I saw my engineering friends playing with in college. No flames involved, so it's all relatively safe. Ah, I now have my first father-daughter project...