Here's why:

* Heat is a form of energy. When you add heat to an object, you're essentially adding energy to the particles that make up that object.

* Particles have kinetic energy. Kinetic energy is the energy of motion. The faster the particles move, the more kinetic energy they have.

* Increased motion: As you add heat, the particles absorb this energy and start moving faster. This increased motion is what we experience as a rise in temperature.

Think of it like this: Imagine the particles as tiny balls bouncing around. When you heat them up, they bounce around faster and with more force!