A liter is a unit of measurement for volume, used to measure how much space a liquid or gas takes up. One liter is about the same as a quart, which means a two-liter bottle of soda holds just a bit more than half a gallon.

Scientists use liters because the metric system makes calculations easier: a liter of water has a mass of about one kilogram and takes up one cubic decimeter of space. This relationship helps chemists, engineers, and researchers work precisely.

You'll see liters everywhere: milk comes in liter containers in many countries, car engines are measured in liters (a 2.5-liter engine), and swimming pools hold thousands of liters of water. When a recipe calls for 250 milliliters of milk, that's one-quarter of a liter.

Today the liter is part of the international metric system used by scientists worldwide and by everyday people in most countries. In America, people still use gallons and quarts more often, but you'll encounter liters often enough, especially in science class, that understanding them is quite useful.