The particle model of matter
All matter is made of particles that are always moving, and how fast they move and how far apart they are is what tells solid, liquid and gas apart.
A worked example
Heating ice adds energy to its particles. They vibrate faster until the solid's rigid arrangement breaks down and the particles can slide past each other as a liquid.
The mistake people actually make
Melting, freezing, boiling and condensing only rearrange existing particles; nothing is added, removed or changed into a different substance. That is what makes a change of state a physical change, not a chemical one.