Here's why:

* Heat is energy. When you add heat to matter, you're essentially adding energy to its particles.

* Energy and motion are linked. The more energy a particle has, the faster it will move.

* Faster particles, higher temperature. The faster the particles move, the higher the temperature of the matter.

Think of it like this: Imagine a bunch of kids playing in a playground. If they're all standing still, it's pretty quiet. But if you give them some energy (maybe by giving them a game to play), they'll start running around, and the playground will be much more lively. The same principle applies to the particles within matter!