An engineer would tell you that a locomotive is driven by steam power. He understands what that means, but you don't. So that is no answer for you. You know what steam is, of course. It is vapor, or water turned to a gas by heat. In turning into gas it expands, or takes up more room. What a fuss steam makes to get out of a teakettle ! If you cork the spout, the steam pushes up the lid—fairly makes it dance. So you can easily see that steam is strong enough to do a good deal of work. It is stronger than you imagine. If you seal a kettle air-tight, so no steam can escape, and let the water go right on boiling, by and by the kettle bursts into little pieces. That gas must have more room. Each tiny atom of that gas is a little hammer that flies around and beats on the walls that hold it in. It hasn't much power alone, but when there are billions of these hammers, all battering at one wall, something has to give way.

An iron boiler is so strong that in it steam or water gas can be made under pressure, or crowded into a very small space. This crowding, of course, increases its explosive, or expansive, power enormously. Then, if a very small opening is left for the gas to escape, it will spend the force of all the steam in the boiler at that opening instead of on the walls. All the gas wants, you see, is to get out where there is more room.

"Well, you may go out," says the engine builder. "But in going out you must push this piston rod that plugs up the opening like a cork." That is just what it does. The steam pushes the piston rod with the force of all the steam in the boiler, like a great hammer, just as you drive a nail in a board. The piston rod pushing forward and falling back as regularly as the pendulum of a clock, turns the drive wheel. And that is what the engineer means when he says a locomotive is "driven'"by steam.