Back at home, I dug around in our garage until I found the basketball inflation insert for our bicycle pump. I put it through the cork. The cork was a about a half-inch too long, so I trimmed the narrow end down with a hack saw (if I'd trimmed the wide end, it wouldn't have sealed the bottle).
The launch site was created by sticking fondue sticks into the ground to keep the bottle pointing upright
Next we filled up the plastic bottles with water...
...and put the bottle on the cork. The tricky part was to make sure that we didn't pull on the pump in such a way to re-aim the bottle and to be sure that we didn't place our heads over the bottle while...
...we were using the bicycle pump to inject air into the bottle.
It usually took two, sometimes three, pumps before the bottle would pop upwards into the sky. We tried different bottle sizes and shapes. I'd say the highest shot went about 1.5 telephone poles high. Too much water in a bottle made it too heavy, and it would only rise about eight feet. No water in a bottle provided little lift, and the bottle popped off the cork only about two feet high.
Our homemade rockets only changed the vehicle and foot traffic patterns a little. The pedestrians thought rocketing soda bottles were The Coolest Things Ever. And those other cars weren't moving anyway.