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1. You take a water bottle upside-down and attach fins so it looks like a rocket.
2. Make a hole in the lid (which will be the bottom of the rocket) just big enough to get a pump nozzle into with minimum leaks. Line it with polyfill to eliminate air escaping.
3. Put a piece of card around the bottle with another piece making a loop (so you can mount it on a launch rod)
4. Take the bottle, put a little water in (about 1/3 full, it makes the whole thing more fun and helps stop air escaping too rapidly)
5. Slide it onto a stick, rod, anything that is smooth and goes straight up out of the ground.
6. Pump it up.
7. Unscrew the bottle (hold the lid, not the bottle
8. Watch it take off or break in a spectacular way!