Video narration
How does heat inside a train become wheels that spin? Think of squeezing a bicycle pump: air pushes back against your hand. Steam can push a movable part in the same way. Heat water until it becomes steam under pressure. A valve directs that steam into a cylinder, where it pushes a piston-the sliding part-one way. Then the steam is directed to push it back. But steam does not directly spin the wheels. A crank, a link that turns the piston's back-and-forth push into rotation, makes the wheel turn. As the piston moves, used steam leaves the cylinder. That clears the way for fresh pressurized steam to push again, keeping the motion going. So how does heat become wheel motion? Boiling water makes pressurized steam; it pushes a piston, and a crank turns that push into spinning wheels.