The candirú, also known as the "toothpick fish," is a tiny, nearly transparent kind of catfish, found in the Amazon river.

The candirú is a parasite that typically enters the gill cavities of larger fish to feed. "A fast, powerful swimmer, the fish is smooth and slimy, with sharp teeth and backward-pointing spines on its gill covers that make it virtually impossible to remove. Still, it's difficult to imagine how even the most agile of fishes could squirm into someone's penis during a brief dip in the water, and in fact one account says women are much more likely to be candirú victims due to the greater dimensions of the female aperture."

Yep. You read that right. If you go swimming in the Amazon river, whatever else you do—don't pee in the water. For years, missionaries, doctors, and other travelers have related stories about these vicious little beasties that sound more like urban legends, cautionary tales about the dangers of traveling in foreign countries. There are several apparently well-documented articles from the 1940's, however. One such article by a U.S. Navy surgeon who operated on three patients who were candirú victims. In one case, the tiny barbed catfish managed to worm its way all the way into one patient's bladder, from which the surgeon extracted it.

Stories about candirú might be a reasonable way to prevent your kid from polluting the swimming pool, though. At least, if you don't mind being responsible for the resulting therapy bills . . .