The tightly packed particles in a solid have little room for movement other than vibration. This rigid particle arrangement gives the solid a definite shape and volume. There is little free space between particles, so the solid is difficult to compress. Since they do not have room for sliding past one another, the solid does not flow.
The solid state is one of the three normal states of matter. In solids, intermolecular forces hold the particles together tightly. Heating the solid increases the vibration of the particles. Once the material reaches the melting point, the solid begins transforming into a liquid. The particles, while still tightly packed, have room for fluid or sliding motion, allowing the material to begin to flow and take the shape of its container. Once it reaches the melting point, the addition of heat energy does not increase the temperature as the energy is breaking the intermolecular forces. Once the material completely melts, the temperature of the liquid begins to rise again until it reaches the vaporization temperature. There, the temperature remains the same until all the particles exist in the gaseous state, where they have the greatest freedom of movement. The gas retains the shape of the container and is easy to compress due to the space between particles.