The steam to power a steam engine is made in a boiler that heats water to make steam. In most places fire heats the boiler. Fuel for the fire may be wood, coal, or petroleum. Nuclear energy or solar energy may be used instead of fire. The steam coming out of the boiler applies the force on a piston. A valve sends the steam to one end of the piston, then the other, to make it move backwards and forwards. The moving piston pushes and pulls the piston rod, crosshead and connecting rod, to turn wheels or drive other machinery. The heavy spinning flywheel smooths out the power from the piston. The governor controls the speed of the engine.