When a man is stimulated, his penis becomes erect. When he has an orgasm, semen comes out from the end of his penis. Semen contains sperm and other substances. How long a man can be stimulated before ejaculating, how long the ejaculation lasts, the strength of the ejaculation, and the amount of semen that comes out, all vary from one man to another. When a man ejaculates, he has a very good feeling in his penis and his groin area (the part of his body where his torso joins the top of his legs). A man can ejaculate after he masturbates, is touched by someone else sexually, and sometimes, even if he is not touched (for example, if he looks at sexy pictures, watches sexy videos or thinks about sex). Ejaculation can also happen without control when a man is sleeping, which is called a wet dream. Boys do not ejaculate until they reach puberty.