Deciduous teeth or milk teeth or temporary teeth are the first set of teeth for most mammals. Humans have 20 of them. The first teeth (called "primary teeth") start to "erupt" (come through the gums of the jaw) when a baby is about 6 months old. When these teeth erupt it can really hurt. Babies chew on things to make the pain better. This is called "teething" (verb: "to teethe"). Most children have all 20 teeth by two or three years of age. 