Ejaculation happens when the body shoots out semen after it has been aroused or stimulated (excited sexually). 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 or is touched by someone else sexually, or 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. Boys do not ejaculate until they reach puberty.
How it happens
Before a man can ejaculate, his penis needs to be stimulated. The penis is usually soft, quite short and hangs down. When a man is stimulated, for example by seeing another naked person, masturbating or being touched by someone else sexually, the inside of his penis fills with blood. This causes it to become longer, thicker, harder and to stand upright, and is called having an erection. Men can be stimulated for different amounts of time before they ejaculate.