Moles, also called nevi, are common growths on the skin. Typically, brown or black, moles are formed by clusters of melanocytes, the pigmented cells that produce skin color. Moles may be raised or flat, and can darken in sunlight, and during puberty or pregnancy. Most moles form in childhood and will grow as the child grows. It is not uncommon for a person to have between 10 to 40 moles by adulthood.