Here's why:

* Heat is energy. When heat is absorbed by a molecule, it gains energy.

* Kinetic Energy. The energy a molecule has due to its motion is called kinetic energy.

* Higher Speed. As the molecule gains more kinetic energy, it moves faster. This increased motion is what we perceive as a higher temperature.

Think of it like this: Imagine a bunch of tiny balls bouncing around. If you heat them up, they'll start bouncing faster and with more energy. This is similar to how molecules behave when they absorb heat.

Important Note: The relationship between heat and molecular speed is not always a linear one. It depends on the type of substance and its state (solid, liquid, gas).