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.