Head lice, also known as nits, are very common in young children and their families. Contrary to popular belief, they don't have anything to do with dirty hair. Head lice are small insects that live in human hair; they are grey and are the size of a sesame seed when fully grown. They are wingless so cannot fly nor can they jump or swim but are spread by climbing from infected hair to the hair of someone else when head-to-head contact is made.