Use a sturdy plastic bottle
Remove the label and rinse the bottle
Make a simple launcher or use a safe bottle rocket launcher
Fill the bottle partially with water
Insert a cork or stopper that fits snugly
Attach a pump to the launcher
Point the bottle away from people, animals, and breakable objects
Pump air into the bottle until the stopper releases
Stand back and observe the launch
Wear eye protection and use the rocket outdoors in an open area