Breaking Intermolecular Bonds
When you heat a liquid, the kinetic energy makes the particles move faster.
When these particles have enough energy to break free from each other, bubbles form in the liquid.
This is boiling.
When you heat a solid, the particles move faster.
They can move around, but are still attached to each other.
This is melting.
The intermolecular bonds are the bonds that hold particles together.