Not steam. An automobile couldn't carry a big iron boiler around. One might say that an automobile is driven by canned sunshine, or gasoline. Gasoline is a refined preparation of petroleum that readily turns into an explosive gas. Ages and ages ago the sun helped make petroleum or vegetable oil, as it helped the plants to grow that turned into coal.

Like steam or water-gas, gasoline-gas pushes things to get out, if shut up. It can be let out in such a way as to push machinery. A very little at a time can be made by feeding a few drops to an electric spark. The spark burns the gasoline that is, makes it so hot it expands into gas. Every time the electric spark passes the jet of gasoline a little explosion takes place. As the gas pushes to escape, it pushes the machinery connected with the wheels. So an automobile is driven by continuous small explosions. When it is stopped the gas-making is stopped, too, and there is no waste of power. And a big tank or boiler is not needed to store the gas. It is made as it is needed, and used up as fast as it is made. When an automobile passes you, you can hear these little explosions that make it go.