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If you believe what you see on TV, women are inscrutable, conniving, hysterical and apt to change their minds without reason or warning. Some women's magazines perpetuate these stereotypes by offering advice on how to entrap men or keep them guessing. And some of the basic differences between men and women can seem a little confusing, depending on your point of view. So it's not surprising that one of the most requested articles in the history of HowStuffWorks is "How Women Work."

The irony is that from conception until the eighth week of gestation, men and women are almost exactly the same. The only difference is at the chromosomal level, deep inside the embryos' cells. Inside every cell of a person's body, DNA is tightly wound into pairs of structures called chromosomes. One pair of chromosomes determines whether the person is male or female. Except in the case of extremely rare abnormalities, a person with two X chromosomes is female, and a person with one X chromosome and one Y chromosome is male. For a few weeks, these chromosomes are all that differentiates male embryos from female embryos.
­Of course, by the time an embryo has grown into an adult wo­man, many attributes make her different from a man. On average, women are shorter and smaller than men are, although women have a higher percentage of body fat. Women have reproductive organs that can support a developing baby and nourish it after its birth. Their blood pressure is lower, and their heart beats faster, even when they're asleep [Source: FDA]. Women also have faster blood flow to their brains and lose less brain tissue as they age than men do [Source: Psychology Today].

­And then, of course, there are hormones, which a lot of people view as a huge difference between men and women. But every person's body, whether it's male or female, uses hormones to regulate and control a wide range of processes. Hormones are the products of the endocrine system, which includes numerous glands located in various parts of the body. For example, two well-known hormones are adrenaline, which comes from the adrenal gland, and insulin, which comes from the pancreas. These and other hormones are vital to the lives and health of both men and women. To learn more about the endocrine system, watch this ADAM animation.

­­Sex hormones, on the other hand, work a little differently in men's and women's bodies. In men, the testes produce the hormone testosterone, which regulates sperm production and causes masculine secondary sex characteristics. In women, the ovaries produce hormones like estrogen and progesterone, which regulate reproductive processes. Men's bodies convert a little testosterone into estrogen, and women's bodies make small amounts of testosterone, so neither hormone is exclusive to one sex or the other.
A man's testosterone levels can fluctuate throughout the day as his body regulates its production of sperm. But a woman's sex hormone levels fluctuate as part of her reproductive cycle, which takes about a month to complete. During a woman's childbearing years, the recurring changes in her hormone levels can cause symptoms like irritability and moodiness, known as premenstrual syndrome (PMS). When a woman reaches perimenopause, her body slows down its production of sex hormones. During the process, her levels of estrogen and progesterone can vary significantly, causing symptoms like hot flashes and trouble sleeping.

Sex hormones can affect a woman's emotions and physiology throughout most of her life. But contrary to some people's perceptions, they're not responsible for every facet of her behavior. In this article, we'll look at some other common perceptions and stereotypes about women as we examine how they work.