Teeth are the hardest organs in the human body, not only for chewing, but also for pronunciation and for maintaining the harmony and beauty of the face. Teeth can be divided into three parts: crown, neck and root, and four parts: enamel, dentin, bone and pulp, according to their composition. The enamel is the hardest component of the tooth tissue, covering the entire outer layer of the crown, and the inner part is the dentin, which is the basic component of the tooth, and the dentin contains the pulp tissue, which is the dental nerve. The dentin is the basic component of the tooth, and within the dentin is the pulp tissue, or nerve. The bone is the part of the tooth that covers the root surface and is weaker than the enamel and dentin.