Puberty is what happens in children's bodies that changes them into adults. After puberty, people are able to make children. A girl who has gone through puberty can become pregnant and have a baby. At puberty, a boy's body begins making sperm, and he starts to be able to ejaculate, which is semen coming out from his penis when he is sexually excited. If he has sexual intercourse (sex) with a girl, he can make her pregnant.