What is a gas? Check out our animation in the States of Matter section.

Take a deep breath. All the air that you just inhaled is made up of lots of different gases – oxygen, nitrogen, carbon dioxide, water vapor, and many, many others. Each gas does different things and behaves in slightly different ways. But all gases have some things in common.

• All gases are made of tiny, tiny groups of atoms called molecules that are moving around very quickly. Temperature (how hot something feels) really tells us how fast the molecules are moving.
• The molecules in a gas bump into each other a lot while they’re moving around. This means that the molecules don’t want to get too close to each other or they’ll hit each other more often. What does this mean? Gases take up some space.
• There is more space between the molecules in gas than in solids or liquids, so it is easy to change the size of the gas.
• Gases like to be as spread out as they can be so the molecules don’t hit each other as much.
• All gases have weight.