Free energy and crystals are important contents in the teaching of basic physics. Crystals are thermodynamically stable solid state systems, which are three-dimensional periodic stacking of large numbers of atoms. The diversity of interactions between atoms indicates that the threedimensional periodic stacking of atoms should be determined by universal factor, which is the law of nature. It is emphasized that the three-dimensional periodic stacking of atoms is the geometric equivalent of the principle of minimum free energy of the system. It is aimed to cultivate students to discover the inner connection of things, and to explore the laws of physics behind natural phenomena.