To change a substance from a solid to a liquid, you need to add energy, specifically heat energy. This process is called melting.

Here's why:

* Solid state: In a solid, particles are closely packed together and vibrate in fixed positions.

* Liquid state: In a liquid, particles have more energy and can move around freely, but they still stay close to each other.

By adding heat, you increase the kinetic energy of the particles, making them vibrate faster. Eventually, they overcome the attractive forces holding them in a fixed position, allowing them to move around more freely, leading to a change of state from solid to liquid.

Example: Ice melts into water when you heat it.