This unique kit allows you to build two different air-and water-powered systems : water-jet propelled cars and hydro-pneumatic powered engines. The hydro-pneumatic system uses a self-contained air-and-water turbine system as a motor. The water-jet propelled cars are a fun outdoor activity, while the self-contained hydro-pneumatic powered models are fun both indoors and outdoors. Compete with your friends to see whose model goes the farthest!