First we filled the bottle with one quarter water.
Then we pumped air into the bottle.
The air causes pressure to build up in the bottle. The more air we pumped in the greater the pressure inside the bottle.
We pumped and pumped until.........
The pressure inside the bottle was too much and the pressure pushed the pump out and rocket off the stand. We had LIFT OFF!!!
Everyone ran as fast as they could to see who would reach the rocket first. We had great fun launching the rocket.