When a man's penis is sexually stimulated for some time, he has an orgasm and semen shoots out from the penis. This is called ejaculation. Boys who have not begun puberty cannot ejaculate. A boy's first ejaculation usually happens about one year after he begins puberty. For some boys, the first ejaculation takes place when they are sleeping. This is called a nocturnal emission or a wet dream. It happens without the boy's control, often when he is having a dream about sex. Scientists believe that wet dreams are the body's way of removing too much semen. A study done of 5,300 white men in the United States in 1948 found that 13% of them had their first ejaculation through a wet dream, This seems to tell us that most boys have their first ejaculation after masturbating. A male masturbates when he rubs his penis to make himself feel good sexually. The study also found that most boys who have their first ejaculation by masturbating do this a year or more earlier than boys who have their first ejaculation through a wet dream. A 2004 survey by a magazine in Canada found that most males (43%) started masturbating when they were about 12 to 13 years old. When a boy begins masturbating, wet dreams often stop happening because semen is taken out from his body regularly.