Steam is the gas that water becomes when it gets hot enough to boil. When you heat water to 212 degrees Fahrenheit (100 degrees Celsius), it transforms from liquid into an invisible gas that rises into the air. What you see rising from a boiling kettle isn't actually steam: it's tiny water droplets that form when the invisible steam cools down and condenses.

Steam contains tremendous energy. When water turns to steam, it expands to take up about 1,600 times more space than it did as liquid, creating powerful pressure. This discovery changed the world. In the 1700s and 1800s, inventors learned to harness steam's power in steam engines, which drove the Industrial Revolution. Steam locomotives pulled trains across continents. Steamships crossed oceans. Steam-powered factories manufactured goods faster than anyone had imagined possible. Even today, most electricity comes from power plants that use steam to spin turbines, whether the steam comes from burning coal, nuclear reactions, or other heat sources.

As a verb, steam means to give off steam or to move using steam power. You can also steam food, which means cooking it with hot steam.

The phrase full steam ahead means moving forward with maximum energy and determination, and comes from the command shouted to steamship engineers. When something happens under its own steam, it succeeds through its own power and effort. If you're feeling angry or need to calm down, you might need to blow off steam, like a steam engine releasing excess pressure through a valve.