The genitals are the main parts of the human body that make men and women different. Some parts of the genitals are outside the body, while other parts are inside. In a man, his external genitals are the penis and the scrotum (a bag that contains the testicles). Inside his body the testicles produce sperm and a substance called testosterone which makes a person grow into a man and to feel like a man. Other glands produce a fluid called semen. The part of a woman's genitals that is outside her body is called the vulva. Inside her body are the vagina, which holds the penis during sexual intercourse; the uterus (womb), in which a baby grows when the woman is pregnant; the ovaries, which produce ova (egg cells) and a substance called estrogen which makes a person grow into a woman and to feel like a woman; and the Fallopian tubes, which connect the ovaries to the uterus.