When puberty begins, between the ages of 10-14 boys will produce millions of sperm cells every day. Each sperm is extremely small: only 1/600 of an inch (0.05 millimeters long).
This development means that the boy is fertile and can now physically make a woman pregnant.
The penis, which usually hangs limp, becomes hard when a man is sexually excited.
The penis fills with blood and it becomes stiff and erect (an erection). The rigidity of the erect penis makes it easier to insert into the woman’s vagina during sexual intercourse.
Semen spurts out of the male’s body through his urethra — this process is called ejaculation. Each time a man ejaculates, it can contain 350 million sperm.
When the man ejaculates during intercourse, semen goes into the woman’s vagina. The sperm swim up through the cervix and through the uterus. If the woman has ovulated (released an egg), a single sperm may penetrate it, and a pregnancy may occur.
To learn more, do the Male Body Quiz