The teaspoon of milky fluid that men ejaculate during orgasm is, for all the drama of its expulsion, mostly just packaging. All but a tiny dewdrop of it is a warm, sugary, protein rich bath that's simply a transport vehicle for its most precious cargo: sperm. In a single ejaculation, the average healthy man expels anywhere from 120 million to 600 million sperm (enough, in theory, to repopulate the world in about two weeks). They're so tiny, though, that they make up only a wee fraction of semen's total volume.

That's why, after a man has had a vasectomy (an operation in which the sperm ducts are severed), he won't notice any change in the amount of semen he ejaculates. About 70 percent ofthe fluid in semen is produced by the seminal vesicles, a pair of sac like organs that empty into the urethra (the pipe through which urine and semen exit the body). The other 30 percent is a clear fluid manufactured by the prostate. Most men ejaculate 3 to 5 milliliters of semen each time they reach orgasm (5 milliliters being roughly a teaspoonful), but as a man gets older, the amount and the force with which it's expelled gradually diminish.

Frequent sex also diminishes semen volume, since it takes the body a few days to replenish its supply. In one Swedish study, 46 healthy, fertile men gave semen samples after one day, and then three days, of sexual abstinence. The researchers noted a "significant increase" in semen volume, as well as sperm counts, the longer these Nordic gentlemen managed to restrain themselves. (If you're having trouble getting your wife pregnant, try not trying quite so often.)