You should know from basic science that the difference in the three phases of matter is the arrangement of their molecules. Solids have compact molecules. Because of this, it is relatively more organized such that it is made up simple, repeating blocks of unit cells. This is called the crystal lattice. When the solid is heated, the kinetic energy of the system increases, thereby increasing frequency of collisions. But since the solid's atoms are compact, they merely vibrate. When the energy gets too high so that phase transformation can be achieved, the space inside the crystal lattice expands and the atoms move farther away from each other. Then, they could turn into a liquid of gaseous phase.
