Erection
An erection () most often refers to an erection of the penis. Erections sometimes occur in the male fetus (the body before birth), and occur naturally in boys from birth onwards. It is normal for the penis to be erect when a male wakes up. This is commonly called morning wood. It is also normal for male teenagers and men to have an erection several times per night when sleeping.  It is widely believed that a man's penis needs to be erect in order for him to take part in sexual intercourse and penetrate his partner's vagina and to ejaculate semen, but all are possible without an erection. In a healthy male, an erection will usually occur if the man is sexually aroused (made sexually excited) or stimulated. This can happen if he is touched sexually, or even if he is not touched – for example, if he sees a naked person or thinks about having sex with someone else. Erections can also occur without thinking about sex or being touched. Most boy teenagers have unexpected erections. This is normal. Erections happen several times during the night. This happens to all healthy adult males of all ages. This also happens when a man wakes up. That is normal too. Tissues in the penis called the corpus spongiosum penis and corpora cavernosa penis fill with blood, causing the penis to become longer, thicker, harder and to stand upright. A man's erection usually stops if he is no longer sexually aroused, or after he has had an orgasm and has ejaculated. An erection will usually not be possible immediately following ejaculation, although adolescent males may be able to achieve erections immediately following ejaculation. The time period during which an erection cannot be repeated following ejaculation is called the refractory period. Animals can have an erection.