Before a male has an orgasm and ejaculates, his penis gets hard. During an erection, the blood vessels in the spongy tissue of the penis fill with blood. This causes the penis to become stiff and stand out from the body.

An erection can happen very quickly, in response to physical stimulation, sexual thoughts, or nothing at all. When a penis gets erect, it also gets longer and wider, and may even change color. Some penises are quite straight when erect, others are slightly curved, or even bent.