Inside a man's body, the testicles make tiny cells called sperm, which are needed for sexual reproduction. The testicles also make a hormone called testosterone. This is a chemical that makes a person grow into a man and feel like a man. The testicles are glands, which are special parts of the body that make chemicals. Other glands that make up the genitals are the prostate, seminal vesicles and bulbourethral glands (also called the Cowper's glands). Together, these glands make a white, sticky fluid called semen that sperm float in. Finally, a man's body contains ducts (tubes) such as the two vasa deferentia or ducta deferentia, which carry sperm out of the testicles; and the urethra, which carries semen through the penis and out of the body. The urethra also carries urine away from the bladder.
In reproduction the job of the penis is to carry sperm from the testicles into a woman's body so that a sperm can join together with the woman's ovum (egg cell) to form a new cell that will grow into a baby. The process of fusion of a sperm and an ovum is called fertilization. A man's penis is usually soft, quite short and hangs down. When a man becomes aroused (sexually excited), the inside of his penis fills with blood. This causes the penis to become bigger, thicker and harder and to stand upright, and is called having an erection. When a man has an erection, he is able to have sexual intercourse by putting his penis into a woman's vagina. During an erection, a valve stops urine from entering the urethra so that only semen flows along it. This is why it is very difficult for a man to urinate when he is having an erection.
Young boys can have erections, but their bodies do not start producing sperm until they have reached puberty. This usually happens when they are in their teens.
The part of a woman's genitals that is on the outside of her body is called the vulva. The main parts of the vulva are two sets of fleshy "lips" called the labia. The outer labia can be seen at the front of the woman's body. On an adult woman, they are generally covered with some hair. Between the outer labia are the inner labia which do not have hair and are very sensitive. At the front of the inner labia is the outside part of the clitoris which is covered by the clitoral hood. During sexual intercourse, the clitoris gives feelings of pleasure to the woman.
Most of a woman's genitals are tucked inside her body. Behind the labia is a tube called the vagina, which cannot be seen on the outside of the body. (People often incorrectly use the word "vagina" to refer to the vulva.) When a man and a woman have sexual intercourse, the man's penis is placed into the woman's vagina. When a woman menstruates (has her menstrual period), blood and other fluid from the uterus pass out from her body through the vagina. If a woman uses a tampon (a specially-made wad of material) to absorb this fluid, she puts it into her vagina. The vagina is also called the birth canal, because when a pregnant woman has a baby, it passes through the vagina.
Like a man, a woman has a tube called the urethra connecting the bladder to the vulva through which urine is removed from the body. The opening of the urethra is between the clitoris and the opening to the vagina.
In many women, the opening of the vagina is partly or completely blocked by a thin tissue called the hymen. Some women are born without a hymen. The hymen usually breaks when a woman has sexual intercourse for the first time, that may cause or not some bleeding depending particularly on each woman. In some cultures, a broken hymen is taken as a sign that a girl has had sex. However, a broken hymen is not a clear sign of sexual intercourse because other activities such as exercise can also cause the hymen to break. In some women, the hymen may remain unbroken even after sex.
At the top of the vagina is the cervix which is a ring of muscle separating the vagina from the uterus or womb. The cervix allows menstrual fluid to flow from the uterus into the vagina, and during sexual intercourse it allows semen from a man's penis to flow from the vagina into the uterus. The uterus is a very muscular and stretchy organ in which babies grow during pregnancy. When a woman is not pregnant, each month the lining of the uterus grows thicker, then breaks down, causing her to menstruate. Connected to the top of the uterus are two Fallopian tubes, on the left and right. These tubes join the uterus to the two ovaries. The ovaries produce ova, or egg cells, that are needed for reproduction. Each month, an ovum travels from one of the ovaries into a Fallopian tube. If, after the woman has had sexual intercourse with a man, the ovum fuses with a sperm and implants in the wall of the uterus, the woman will become pregnant. The ovaries also produce the female hormone estrogen which causes a person to grow into a woman and to feel like a woman.
Young girls do not start producing ova and menstruating until they have reached puberty. Like boys, this usually happens in their teens. Once a woman starts menstruating, she is capable of becoming pregnant.
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.
Sometimes, a man and a woman may have sexual intercourse because it gives them pleasure, without wishing for the woman to become pregnant. They may try to stop fertilization from happening by using methods of contraception or birth control. Common methods of birth control include the man wearing a condom on his penis, and the woman taking birth control pills that stop her ovaries from releasing egg cells.