* Kinetic Molecular Theory: Air is made up of tiny molecules that are constantly moving. When you heat the air, you increase the kinetic energy of these molecules.

* Increased Movement: The molecules move faster and collide with each other and the walls of their container more frequently and with more force.

* Expansion: This increased pressure from the moving molecules causes the air to expand, increasing its volume.

Think of it like this: Imagine a balloon. When you heat the air inside the balloon, the air molecules move faster, pushing against the inside of the balloon and making it expand.

This principle is known as Charles's Law, which states that the volume of a gas is directly proportional to its absolute temperature when pressure is held constant.