Gather a plastic bottle, water, a cork or rubber stopper, a bicycle pump, tape, and cardboard or plastic for fins
Cut and attach fins to the bottle body
Add a nose cone to the bottle bottom if desired
Fill the bottle about one-third with water
Insert the stopper tightly into the bottle opening
Insert the pump needle or tube through the stopper
Place the bottle upright in an open outdoor area
Secure the bottle so it stays stable
Pump air into the bottle
Release the stopper or trigger the launch mechanism
Stand back immediately after pressurizing
Recover the rocket after it lands