A water-and-air rocket from a plastic bottle. Reaches 50+ feet.

Materials

• 2-liter plastic bottle
• Cork that fits the bottle neck snugly
• Bike pump with needle valve (the kind for inflating sports balls)
• Water

Steps

1. Fill bottle 1/3 with water
2. Insert cork with needle valve through it (needle pushes into bottle neck)
3. Flip bottle onto launchpad (three sticks for tripod stand)
4. Pump air into bottle through the valve
5. Pressure eventually pushes cork out; water exits; rocket flies

Safety: clear area, no humans in trajectory, protective glasses. Science-class-level physics demonstration with memorable results.