Time is the ongoing sequence of moments that moves from the past through the present and into the future. It's what lets us measure how long things take, like the two weeks until your birthday or the 45 minutes of a soccer game. Without time, everything would happen at once, which is impossible to even imagine.

We measure time in seconds, minutes, hours, days, and years, but time itself keeps flowing whether we measure it or not. When you're having fun, time seems to fly by. When you're bored or waiting for something exciting, time seems to crawl. Yet the clock ticks at exactly the same speed either way.

Time also means a specific moment or period. “Once upon a time” starts a story in the past. When you say “I don't have time right now,” you mean that moment is already filled with something else. A difficult time might mean a challenging period in someone's life.

As a verb, time means to measure how long something takes. You might time yourself running a lap or time how long it takes water to boil.

Scientists have discovered that time is stranger than it appears. Einstein showed that time can move at different speeds depending on how fast you're traveling and how strong gravity is around you. An astronaut orbiting Earth in a spacecraft ages slightly slower than people on the ground, though the difference is tiny. Time is one of the universe's deepest mysteries, and physicists still debate exactly what it is and why it flows in only one direction.