The first model was one we found through the collective wisdom of the Web. We simply poked a hole in the cap of the bottle.
Then, we filled the bottle with water, and replaced the cap. Squirt gun number one was ready to go. It's a simple point-and-squeeze design.
The second model, which we found in Pop Bottle Science, by Lynn Brunelle, was a little more complicated. We poked a hole in the bottle cap again, but this time we also poked a hole near the bottom of the bottle.
Then, we stretched the mouth of a balloon over the bottle opening, letting the balloon dangle down into the bottle.
We filled the balloon with water.
When it was pretty well full, we covered the hole in the bottle, and put on the cap. Uncovering the hole, allowed air into the bottle, and the pressure on the balloon caused water to shoot out through the hole in the cap.
It sounded good, but did not perform quite as well as the simpler model, and was the definite loser in a face to face competition.
It's great to be a homeschooler.