Pretty simple kids toy. Attach a balloon and watch it scoot across the floor! (Sorry, some extra parts required)The wheels I used were from an older meccano set (a knock off at that) where the wheels just pop onto a metal axel. The bushings for the metal axel are 3mm square holes, while the rest of the axel cavity is hollowed out to 4mm square.Just add wheels, a ballon, inflate and watch it go!One extra note. The exhaust port on the back is currently a 4mm X 4mm square hole. I've found that making the hole larger will give you more thrust over a shorter duration, so making it slightly larger is probably better. (maybe 5 x 5 is a good place to start?