The penis is an organ of the male reproductive system, which is found in mammals.

The penis is an organ of the male reproductive system, which is found in mammals. It has a number of functions, including urine excretion and sexual reproduction. The penis contains erectile tissue that becomes engorged with blood during sexual arousal, turning it into a rigid structure that can be inserted into the vagina for sexual intercourse. When not erect, the penis is flaccid and hangs down from the body. The average length of an adult human penis is around 13.12cm (5.2 inches), but it can range from 9cm to 17cm (3.5 inches to 6.7 inches).

The tip of the penis resembles a mushroom because it contains spongy tissue that fills with blood when aroused, making it firm and erect.