A steam engine is a machine that burns fuel (usually coal or wood) to heat water until it turns into steam, then uses the power of that expanding steam to move pistons or turn wheels. Picture a teakettle on a stove: as the water inside gets hotter and hotter, it turns to steam that pushes out with enough force to make the lid rattle and shake. A steam engine captures that same pushing force and uses it to do work.

The invention of practical steam engines in the 1700s changed human civilization dramatically. Before steam power, people relied on muscle power (human or animal), water wheels, or windmills to run machines. Steam engines could work anywhere, anytime, regardless of whether a river was flowing or the wind was blowing. Factories used steam engines to power looms and other machinery. Steam locomotives pulled trains across continents, connecting cities that had been weeks apart by wagon. Steamships crossed oceans faster and more reliably than sailing vessels ever could.

James Watt created the most famous early design in the 1760s, though others had built simpler versions before him. His improvements made steam engines efficient enough for widespread use. By the 1800s, the rhythmic chuff-chuff-chuff of steam engines became the soundtrack of the Industrial Revolution.

Today, most engines run on gasoline or electricity, but steam engines still operate in some power plants (using steam to spin turbines that generate electricity) and on historic railroads, where people can experience what travel was like 150 years ago.