Here's why:
* Heat is energy. When you add heat to a substance, you're adding energy to the molecules within it.
* Energy makes them move. This extra energy causes the molecules to vibrate and move around more rapidly.
* Increased movement can lead to expansion. While the molecules themselves don't get bigger, the increased movement and vibrations can cause them to spread out a bit more, leading to an overall expansion of the substance (like when water boils and turns into steam).
Think of it like this: Imagine a bunch of people standing close together in a room. If you give them all a lot of energy, they'll start moving around more and might bump into each other, pushing each other further apart.
