Here's why:

* Increased Kinetic Energy: Heat is a form of energy. When you heat something, you increase the kinetic energy of its molecules. They move faster and vibrate more vigorously.

* Increased Spacing: As molecules move faster, they take up more space, causing the substance to expand.

* Exceptions: There are some exceptions to this rule. For example, water expands when it freezes because the structure of ice crystals takes up more space than liquid water.

Let me know if you have any other questions!