Here's why:

* Heat: When you add enough heat to a solid, the particles within it gain energy and start to vibrate faster.

* Breaking Bonds: As the vibrations increase, the bonds holding the particles in a rigid structure weaken and eventually break.

* Free Movement: With the bonds broken, the particles can move more freely, giving the substance a liquid state.

Think of it like this:

* Solid: Imagine a tightly packed crowd of people standing still, representing the particles in a solid.

* Liquid: Now imagine the crowd becoming more energetic, moving around, and bumping into each other, representing the particles in a liquid.

The specific temperature at which a solid melts is called its melting point.