The penis of a boy gets hard so that it can easily go into the girls vagina. the general idea is that when he does this and moves it back and forth he has an orgasm when this happens (assuming he has reached puberty) he ejaculates semen from his penis into the girl. If the girl has reached puberty and is having her menstrual cycle there is a chance that the sperm from the boys ejaculation will reach an ovum in the girl and make her pregnant. I would strongly suggest that if you are asking this question because this is what is happening that you get some information on reproduction and on contraception (ways to stop reproduction) sex may be fun but becoming a mother when you don't want to be definitely is not.
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