Newton's Third Law: For every action, there is an equal but opposite reaction. We tested which performs best, bottles with 3/4 water and 1/4 air or 1/4 water and 3/4 air. We used two sizes of bottles. The pump adds air pressure and the water acts as a fuel. The more air you have, the more of a thrust you have when the fuel is released. This is a fun way to show this law, and it can be as involved as you want it. Make it a huge PBL where the students build and decorate their rockets and find data on weight and other factors, or make it a small investigaton where the teacher prepares the entire lab. This was a prepared lab.