* Gases are compressible: Unlike solids and liquids, gases have very weak intermolecular forces. This means their molecules are far apart and can be squeezed closer together.

* Volume depends on pressure and temperature: The volume of a gas is directly related to the pressure and temperature it's exposed to.

* Pressure: Increasing the pressure on a gas forces the molecules closer together, decreasing the volume.

* Temperature: Increasing the temperature of a gas gives the molecules more kinetic energy, causing them to move faster and spread out, increasing the volume.

Example: Think about a balloon. When you blow air into it, you are increasing the pressure inside. The gas molecules get squeezed closer, increasing the pressure until the balloon expands.

Key takeaway: The volume of a gas is not fixed and can change depending on the conditions it's in.