The sex organs or genitals are used for sexual reproduction and for sexual intercourse. For sexual reproduction to happen, a man and a woman need to have sexual intercourse with each other. A man's penis becomes erect when he is aroused, for example when he sees a naked woman or is touched by her. When a woman is aroused, her clitoris and vulva also swell, and the inside of her vagina produces mucus, a substance that makes it slippery. To reproduce, the man places his penis inside the woman's vagina and moves it in and out (a movement called thrusting), while the woman moves her hips back and forth, or in a circular motion. The friction caused by this movement, together with the warmth and pressure of the vagina, causes the man to have pleasurable feelings in the penis. As intercourse continues, these feelings grow stronger and stronger until the man reaches a sexual climax called an orgasm. At this point, the man's penis spasms and then contracts strongly again and again to push semen through the urethra. The semen then ejaculates or shoots out from the end of the penis into the woman's vagina. The woman may also have an orgasm, which causes the vagina to tense up and relax repeatedly. Scientists are not sure why women have orgasms. Some believe that the orgasm helps the sperm in the man's semen to swim up the vagina into the cervix, so that it is more likely that a sperm cell will fuse with an egg cell. Others think that the female orgasm causes the vagina to grip the penis more tightly, which makes sex more exciting for the man and causes him to ejaculate more quickly or to produce more semen; or that it encourages a woman to have sex more often as it feels good. Both of these events would make pregnancy more likely to happen. After the man has ejaculated, the blood flows out of his penis and it becomes smaller and soft again.