Solids particles are packed closely together. The particles are only able to vibrate about fixed positions; they can’t move around because they have strong force of attraction between them, which keep them together. Heat is usually the reason why solids vibrate. When you heat a solid the energy provided by the heat source makes the solid vibrate faster and faster. It vibrates so fast that it causes the force of attraction in between the particles become no longer strong enough to hold them together, and here the solid becomes a liquid.
