When it comes to men and sex , they're pretty easy to figure out. Men's penises become erect when they're sexually aroused, and they release ejaculate when they've had an orgasm. Because women's arousal is harder to detect, and because women's orgasms aren't essential to reproduction, many people -- both men and women -- assumed that sex was something women put up with to have babies, not something that could be enjoyed. Now, of course, there are plenty of resources, from Cosmopolitan magazine to sex therapists, who are ready to explain how a woman can gain pleasure from sex. For one thing, female arousal is now better understood.

Sexual intercourse , also called coitus , or copulation , reproductive act in which the male reproductive organ in humans and other higher animals enters the female reproductive tract. If the reproductive act is complete, sperm cells are passed from the male body into the female, in the process fertilizing the female egg and forming a new organism. In some vertebrates, such as fish , eggs are laid outside of the body and fertilized externally. To accomplish internal copulation certain body and organic adaptations are necessary. In the human male, the penis serves both excretory and reproductive functions. During intercourse, the blood flow is temporarily increased and trapped in the penis so that it becomes enlarged and elevated, a condition known as erection. Erection changes the normally soft and flaccid organ to one of greater size and rigidity to permit easier penetration into the reproductive tract of the female.

According to Don and Debra MacLeod's Lube Jobs: A Woman's Guide to Great Maintenance Sex , maintaining the sexual spark is akin to the way we maintain a car: we don't get to do the big check-up, so we do small upkeep to keep the engines running. The car maintenance metaphor is not very inspiring. It lacks poetry. What people want when they complain about the listlessness of their sex lives is to recapture the poetics of sex, the connection, the renewal, the playfulness, the feeling of aliveness, a sense of lifting themselves from the mundane -- not maintenance. All quick fixes, instructions, are intended to save us from our hectic lives.

Great sex goes beyond titillating tricks and quivering orgasms. Ian Kerner. Sex affects your brain in the same way drugs do. This is one of the chemicals that give you that exuberant feeling. You can thank your brain for all those lovely warm fuzzy feelings that emanate from sexual pleasure.