Matter can appear in three states of aggregation: solid, liquid and gas.

In the solid state bodies have an almost invariable volume (incompressibility) due to the fact that their particles (atoms, ions and molecules) are practically in contact with each other and so cannot get any nearer.

The shape of solids is also invariable, because their particles are perfectly arranged and occupy fixed positions in repeated three-dimensional structures called crystals. The particles are not static in their positions but vibrate ceaselessly, the more intensely the greater the temperature. If this becomes sufficiently high (melting point) the particles lose their fixed positions and even though they continue to be close together, the crystalline structure, exclusive to solids, disappears and they change into liquids.