Snap Build a Fuel Cell Toy Car that Runs with Salt Water The Salt Water Fuel Cell Car gives children a chance to learn about new forms of clean energy, while building and powering their very own toy: Just add saltwater and go! After activating the fuel cell module with a saltwater mixture, the magnesium metal sheet (3 sheets included) can operate the car for about 5-7 hours continuously. If you want to park the vehicle, simply take out the fuel cell module and rinse with tap water and dry.