* Solids have a fixed shape and volume because their particles are tightly packed and held together by strong forces.

* Liquids have a fixed volume but can take the shape of their container because their particles are more loosely packed and can move around more freely.

Melting is the process where the particles in a solid gain enough energy (usually by adding heat) to overcome the forces holding them together, allowing them to move more freely and transition into a liquid state.