|This cut view of a child's skull shows permanent teeth located above and below the deciduous teeth prior to exfoliation. The deciduous mandibular central incisors have already been exfoliated.|
Deciduous teeth, otherwise known as milk teeth, baby teeth, temporary teeth and primary teeth, are the first set of teeth in the growth development of humans and many other mammals. In some Asian countries they are referred to as fall teeth as they will eventually fall out (see deciduous), while in almost all European languages they are called milk teeth. They develop during the embryonic stage of development and erupt—that is, they become visible in the mouth—during infancy. They are usually lost and replaced by permanent teeth, but in the absence of permanent replacements, they can remain functional for many years.
Deciduous teeth start to form during the embryo phase of pregnancy. The development of deciduous teeth starts at the sixth week of development as the dental lamina. This process starts at the midline and then spreads back into the posterior region. By the time the embryo is eight weeks old, there are ten areas on the upper and lower arches that will eventually become the deciduous dentition. These teeth will continue to form until they erupt in the mouth. In the deciduous dentition there are a total of twenty teeth: five per quadrant and ten per arch. The eruption of these teeth ("teething") begins at the age of six months and continues until twenty-five to thirty-three months of age. Usually, the first teeth seen in the mouth are the mandibular centrals and the last are the maxillary second molars.
The deciduous dentition is made up of central incisors, lateral incisors, canines, first molars, and secondary molars; there is one in each quadrant, making a total of four of each tooth. All of these are gradually replaced with a permanent counterpart except for the first and second molars; they are replaced by premolars. The replacement of deciduous teeth begins around age six, when the permanent teeth start to appear in the mouth, resulting in mixed dentition. The erupting permanent teeth causes root resorption, where the permanent teeth push on the roots of the deciduous teeth, causing the roots to be dissolved and become absorbed by the forming permanent teeth. The process of shedding deciduous teeth and their replacement by permanent teeth is called exfoliation. This may last from age six to age twelve. By age twelve there usually are only permanent teeth remaining.
Teething age of deciduous teeth:
- Central incisors : 6–12 months
- Lateral incisors : 9–16 months
- First molars : 13–19 months
- Canine teeth : 16–23 months
- Second molars : 22–33 months
Deciduous teeth are considered essential in the development of the oral cavity by dental researchers and dentists. The permanent teeth replacements develop from the same teeth buds as the deciduous teeth, which provide guides for permanent teeth eruptions. Also the muscles of the jaw and the formation of the jaw bones depend on the primary teeth to maintain proper spacing for permanent teeth. The roots of deciduous teeth provide an opening for the permanent teeth to erupt. The deciduous teeth are also needed for proper development of a child's speech and chewing of food.
Cultural traditions 
Various cultures have customs relating to the loss of deciduous teeth. These were most commonly associated with mice or other rodents on account of their sharp, everlasting teeth.
In Britain, lost teeth were commonly burnt to destroy them. This was partly for religious reasons connected with the Last Judgement and partly for fear of what might happen if an animal got them. A rhyme might be said as a blessing:
Old tooth, new tooth
Pray God send me a new tooth
The legend of the tooth fairy is that of a fairy that gives a child money and/or gifts in exchange for a baby tooth that has fallen out. Children typically place the tooth under their pillow at night. The fairy is said to take the tooth from under the pillow and replace it with money once they have fallen asleep. In some places in Australia, Sweden and Norway, the children put the tooth in a glass of water.
Tooth tradition present in the United States sometimes comes under different names. A Ratón Pérez appeared in the tale of the Vain Little Mouse. The Ratoncito Pérez was used by Colgate marketing in Venezuela and Spain. In Italy, the Tooth Fairy (Fatina) is also often replaced by a small mouse (topino). In France and in French-speaking Belgium, this character is called la petite souris ("The Little Mouse"). From parts of lowland Scotland comes a tradition similar to the fairy mouse: a white fairy rat who purchases the teeth with coins. In medieval Scandinavia there was a tradition, surviving to the present day in Iceland, of tannfé ('tooth-money'), a gift to a child when it cuts its first tooth.
In Turkey, Cyprus and Greece, children traditionally throw their fallen "milk teeth" onto the roof of their house while making a wish. Similarly, in some Asian countries, such as India, Korea, Vietnam, and the Philippines, when a child loses a tooth, the usual custom is that he or she should throw it onto the roof if it came from the lower jaw, or into the space beneath the floor if it came from the upper jaw. While doing this, the child shouts a request for the tooth to be replaced with the tooth of a mouse. This tradition is based on the fact that the teeth of mice grow for their entire lives, a characteristic of all rodents. In Japan, a different variation calls for lost upper teeth to be thrown straight down to the ground and lower teeth straight up into the air; the idea is that incoming teeth will grow in straight.
In parts of India, young children offer their discarded baby teeth to the sun, sometimes wrapped in a tiny rag of cotton turf[clarification needed]. In southern India, children bury their teeth in the soil hoping for a newer one to grow. The Sri Lankan tradition is to throw the milk teeth onto the roof or a tree in the presence of a squirrel (Funambulus palmarum). The child then tells the squirrel to take the old tooth in return for a new one. Some parts of China follow a similar tradition by throwing the teeth from the lower jaw onto the roof and burying the teeth from the upper jaw underground, as a symbol of urging the permanent teeth to grow faster towards the right direction.
The tradition of throwing a baby tooth up into the sky to the sun or to Allah and asking for a better tooth to replace it is common in Middle Eastern countries (including Iraq, Jordan, Egypt and Sudan). It may originate in a pre-Islamic offering and certainly dates back to at least the 13th century, when Izz bin Hibat Allah Al Hadid mentions it.
See also 
- Steve Roud (2006), "Teeth: disposal of", The Penguin Guide to the Superstitions of Britain and Ireland, ISBN 978-0-14-194162-2
- ¡Producto Registrado!: Agosto 1998: Centuria Dental.
- Cleasby, Richard and Gudbrand Vigfusson, An Icelandic-English Dictionary, 2nd edn by William A. Craigie (Oxford: Oxford University Press, 1957), s.v. tannfé (first edition available at http://www.ling.upenn.edu/~kurisuto/germanic/oi_cleasbyvigfusson_about.html).
- Al Hamdani, Muwaffak and Wenzel, Marian. "The Worm in the Tooth", Folklore, 1966, vol. 77, pp. 60-64.
- Gray anatomy skull of 6-year old child showing both sets of teeth
- Primary dentition - Baby teeth chart Morphology and eruption time (teething age) of primary teeth.