Here's why:

* Heat is energy. When you add heat to a substance, you're essentially giving its particles more energy.

* More energy means more movement. This extra energy causes the particles to vibrate and move around more rapidly.

* Faster movement leads to expansion. The particles are colliding more often and with greater force, pushing each other farther apart. This is why liquids expand when heated, and why gases expand even more dramatically.

Let me know if you have any other questions!