Many parents respond when their children come back from school and say, “The school did a physical exam and recommended fossa closure.” So, what is fossa closure all about? Is it necessary? A fossa is an uneven depression on the occlusal surface of a large tooth (molar), which is a defect formed by the fusion of the tooth cusps during tooth development. The sulcus of each tooth varies in shape and depth, and is a suitable place for food residues to remain and bacteria to grow and multiply. If the sulcus is very deep, then the bacteria in these hidden narrow sulcus, using the residual food sugars to reproduce and grow, and in the process of metabolism produce acidic substances, corrosion of teeth, easily occur caries, medical called this caries as sulcus caries. The data shows that more than 90% of the caries (worm teeth) of our teenagers occur in the sulcus part, so the risk of caries in deep sulcus is very high. The technique of child sulcus closure is a technique that can effectively strengthen the anti-cavity ability of teeth for children when they are developing. Specifically, a polymer composite resin material is applied in the sulcus of children’s teeth, and the liquid resin cures and hardens after entering the sulcus, forming a protective barrier, just like putting a protective coat on the defective teeth to protect them from the erosion of food and bacteria, thus enhancing the anti-caries ability of teeth. Vulcanization is mainly aimed at the prevention of caries (worm teeth), so it is mainly for the newly erupted permanent molars of children and adolescents. Sixth-aged teeth are the teeth that bear the most important function of chewing, because of early eruption, if not pay attention to protection, it is easy to suffer from caries (insect tooth), so it becomes the main target of the sulcus closure. Usually, six-year-old teeth are fully erupted at the age of 7-8, which is the best age for closure. With the eruption of other permanent teeth, several more sealings will be performed. Due to wear and tear, bonding, etc., the sealant material may also fall out, so it is important to visit a professional dentist regularly for checkups and replenishment if needed. Teeth are important and need to be protected as early as possible!