The particles in a material move faster when heated. This increased movement causes the particles to spread out, which can make the material expand and appear bigger.

Here's a breakdown:

* Heat energy: When you heat a material, you're adding energy to its particles.

* Increased movement: This added energy causes the particles to vibrate and move faster.

* Expansion: As the particles move faster, they collide more frequently and with greater force, pushing each other further apart.

* Apparent size increase: This increased spacing between particles leads to an overall expansion of the material, making it appear larger.

Important note: While the particles themselves don't actually get bigger, the space between them increases, making the material appear larger.