The soda bottle car is driven by compressed air and is an excellent example for demonstrating Newton’s 3rd law of motion. The soda bottle is pumped up to about 60 PSI with air using a portable air pump. The bottles are able to withstand higher pressures but I keep it to this range. On a good run without hitting a wall it can travel well over 100 feet. I have made a few versions of the cart, starting with Leggos and Kinex. Any design of car simply needs to be able to hold the soda bottle horizontal and hopefully steers straight. The hole in the soda bottle cap is 1/16th of an inch. The needle inflator stem fits into very snuggly but will loosen with use, when it starts losing too much air, replace it with a new cap.