So let's start with a basic definition: Kinetic theory of matter
This is all matter is made up of atoms and molecules that are constantly moving.
When heat is added to a substance, the molecules and atoms vibrate faster. As atoms vibrate faster, the space between atoms increases. The motion and spacing of the particles determines the state of matter of the substance. The end result of increased molecular motion is that the object expands and takes up more space.
Hope this helps you!