Consider the following-

All the matters are made up of tiny particles. The particles of matter attract each other. It is known as intermolecular force of attraction. This attraction varies from one state to another. For example solids have the highest force of attraction and the gases have the least force of attraction. In the solids the space between the molecules is less. For this reason the attraction between the molecules is high, that is why the solids do not change their structure.