Each tooth in the adult human features three primary components: the crown, the neck, and the root. The crown is the portion of the tooth which projects above the gumline, the neck is the portion of the tooth which is surrounded by the gum tissue, and the root is the conical stem (or stems) which project into the jawbone and anchor the tooth. These three portions of the tooth compose the tooth as a whole, and are covered by a yellowish material called |dentin|. A white |enamel| covers the crown, while a layer of |cementum| covers the neck and root of each tooth. The |dentin| contains a high percentage of calcium and is a harder substance even than |compact bone| tissue. Harder and more durable than this, however, is |enamel|, which is the hardest substance in the human body. |Teeth| may be either deciduous or permanent. Deciduous |teeth|, or baby |teeth|, are the impermanent |teeth| which grow in (erupt) starting in the first few months of life. Twenty of these |teeth| grow into the dental arcade of the child. These are shed and replaced by the permanent |teeth|, of which there are thirty-two in the average adult. Dental diseases are among the most common, yet easily prevented, suffered by in society. Most (though not all) are due to poor dental hygiene. When the neck of the tooth is exposed by receding gums (caused by advanced age and some diseases like gingivitis) it is subject to damage by bacteria. Additionally, poor dental hygiene may cause cavities to form in the crown, requiring dental fillings of porcelain, gold, or silver amalgam. The American Dental Association recommends use of a fluoridated |toothpaste|, brushing at least twice a day (but preferably after each meal), and flossing between |teeth| regularly. In addition, one is urged to visit a dentist every six months for a routine check-up. Since calcium and phosphorus are important in the maintenance of |teeth| and bone, it is suggested that one's diet reflect these needs, based upon the U.S. Recommended Daily Allowances for these |minerals|.