Mass is the amount of matter in something. Every object has mass: a baseball, a bicycle, a building, even a single grain of sand. Mass measures how much stuff makes up an object, whether it's on Earth, floating in space, or sitting on the moon.

Mass is different from weight, though people often confuse the two. Your mass stays the same everywhere in the universe, but your weight changes depending on gravity. An astronaut who has a mass of 50 kilograms on Earth still has a mass of 50 kilograms on the moon, but weighs much less there because the moon's gravity is weaker. Mass is what you're made of; weight is how hard gravity pulls on that mass.

Scientists measure mass in grams and kilograms (or in slugs in the U.S. customary system). A paper clip has a mass of about one gram. A liter of water has a mass of one kilogram. Your own mass probably ranges between 30 and 50 kilograms, depending on your age and size.

Objects with more mass are harder to move and harder to stop once they're moving. This is why pushing a shopping cart full of groceries takes more effort than pushing an empty one. The concept of mass helps explain everything from why planets orbit the sun to why a bowling ball knocks down pins more effectively than a tennis ball.