That three-letter word that can make you both blush and say "aahhh" is more than a simple act of lust. Sex has been around since, oh yeah, nearly the beginning of time. And since then scientists and enthusiasts alike have delved into understanding and even improving the act and its outcomes. Here's a look at some of the coolest facts about S-E-X.
Let's get down to business about the facts of life: Sex is made for mixing and mingling genes. These genes are carried by the male and female gamete cells, the sperm and eggs. Most of the time, a woman releases one egg per month, but men are much more prolific. In a single ejaculation, a guy sends between 30 million and 750 million sperm swimming toward that egg.
Human males have nothing on pigs, though. A single swine ejaculate contains about 8 billion sperm cells.
Sex on the brain
Humans start to think about sex relatively early in our life spans. By age 19, about 70 percent of American teenagers have had sex.
Unintended pregnancies are all too common, but the stars do have to align to get a single sperm to a ready egg. Women are fertile for about three to six days each month, depending on their menstrual cycle. Sperm can hang around in the reproductive tract for up to five days and survive, but it's rare for them to remain viable for more than two days or so.
Trying to keep these stars from aligning may be tough, though. With typical use, fertility-awareness methods of birth control (when a couple avoids sex during fertile days) result in 24 pregnancies for every 100 couples who use these methods each year.
Ovulation heats up a woman's body by as much as half a degree Fahrenheit. Before ovulation, most women run between 96 and 98 degrees F (35.5 to 36.6 degrees Celsius). Right after ovulation, body temperature goes up to around 97 to 99 degrees F (36.1 to 37.2 degrees C). The most effective fertility-awareness methods of birth control require daily temperature-taking to detect ovulation.
Bestiality is risky
Bestiality is generally frowned upon. Here's one more reason to abstain: A 2011 study found that sex with animals, such as chickens, pigs and horses, is linked to penile cancer, perhaps because microtrauma to the penis during these acts lets foreign microorganisms in. Some microorganisms, such as the human papilloma virus (HPV), can cause cancer in humans.
It's no accident that nipples are erogenous zones. Brain-imaging research on women has shown that sensory signals from the nipples end up in the same area of the brain that stimulation from the vagina, cervix and clitoris do. 
Oh how exciting!
Humans sometimes go to strange lengths for sex. Take the alleged aphrodisiac Spanish fly. It's a ground-up bug called a blister beetle that contains the acid cantharidin. When taken and excreted, it causes a burning sensation in the urethra that apparently passes for sexual excitement in some circles. Oh, and the powder is toxic. [Top 10 Aphrodisiacs]
Learning about sex makes you more likely to go out and do it, right? Nope. According research published in 2012 by the nonprofit Guttmacher Institute, any sex education at all delays teen sex.
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Stephanie Pappas is a contributing writer for Live Science, covering topics ranging from geoscience to archaeology to the human brain and behavior. She was previously a senior writer for Live Science but is now a freelancer based in Denver, Colorado, and regularly contributes to Scientific American and The Monitor, the monthly magazine of the American Psychological Association. Stephanie received a bachelor's degree in psychology from the University of South Carolina and a graduate certificate in science communication from the University of California, Santa Cruz.