Each tooth is anchored in the alveolar bone of the jaw through its root(s) as a tree is anchored to the soil through its roots. The portion of the tooth that is exposed in the oral cavity is called the crown of the tooth while the portion of the tooth that is buried inside the alveolar bone is called the root(s) of the tooth. How strong the tree stands in the air depends on how much roots are embedded into the soil. If the soil gradually gets lost and more roots are exposed onto the ground, the tree may fall down as too little soil is holding it up. This situation is similar to a tooth. When the alveolar bone gradually gets lost and more of the tooth root is now exposed, the tooth will become loose. Eventually the person will lose that tooth not because there is any decay on the tooth itself but because there is no bone to hold the tooth in place. The phenomenon of losing the alveolar bone, which function is to hold the teeth in place, is called periodontal disease (or more commonly known as gum disease). While cavity makes the teeth rotten and decayed, periodontal disease makes the teeth loose (mobile) in the oral cavity. Both problems can eventually lead to teeth loss if left untreated.