In reproduction the job of the penis is to carry sperm from the testicles into a woman's body so that a sperm can join together with the woman's ovum (egg cell) to form a new cell that will grow into a baby. The process of fusion of a sperm and an ovum is called fertilization. A man's penis is usually soft, quite short and hangs down. When a man becomes aroused (sexually excited), the inside of his penis fills with blood. This causes the penis to become bigger, thicker and harder and to stand upright, and is called having an erection. When a man has an erection, he is able to have sexual intercourse by putting his penis into a woman's vagina. During an erection, a valve stops urine from entering the urethra so that only semen flows along it. This is why it is very difficult for a man to urinate when he is having an erection.