By Hampton Sides

How many times can a stone skip?
— Adrian McCloskey, Rapid City, South Dakota

Virtually every culture has a term for stone-skipping. The English call it "ducks and drakes"; to Danes, it's "smutting." Eskimos skip rocks on ice, Bedouins on smooth sand. Currently, The Guinness Book of World Records accords the title to Jerdone Coleman McGhee, a Texas engineer who in 1992 scored an incredible 38 skips on the Blanco River. Author of The Secrets of Stone Skipping, McGhee has consulted with MIT engineering students, who've used strobe photography to analyze the fluid dynamics of a skip (which, by the way, involves releasing the stone almost parallel to the water, with enough velocity and spin to create a small wave on impact and then bounce off that wave and go airborne again). McGhee's mark is not without controversy, however. Purists insist that skipping prowess undergoes its true test only in a sanctioned competition, when the heat is on. The granddaddy of all such skip-offs is held every Fourth of July on Michigan's Mackinac Island. It was here in 1977 that John Kolar earned the all-time Mackinac record of "24-plus-infinity" (his stone vanished ominously into fog after two dozen skips). Kolar calls Mackinac, with its variable weather, boat traffic, and nearly constant chop, the "Wimbledon Centre Court of skipping — it quickly weeds out the competition." Indeed, when Jerdone McGhee made the pilgrimage to Mackinac, he was duly humbled by a score of only 17.

Why is yawning contagious?
— Barbara Billings, Eugene, Oregon

First, let us dispense with the antiquated notion that a yawn is the body's attempt to pump extra oxygen into our fatigued systems. Scientists have shown that a person in an oxygen chamber yawns just as much as anyone else. No, yawning seems to be tied to more primal purposes, with contagion at the center of it. A semiautomatic reflex that originates in the brain stem and marks transitions from one mental state to another, yawning is widespread throughout the animal kingdom, particularly among carnivores. Watch a pride of lions: When one cat opens its mouth, pretty soon there's an outbreak. While the precise mechanism for its contagiousness remains a mystery, yawning appears to be a piece of ancient genetic wiring meant to help synchronize clan behavior, a form of prelinguistic communication indicating it is time to move on to the next big thing. Think of it as a way for, say, a Neanderthal to let his comrades know that the nap's over, let's go bag us a woolly mammoth.

What is that smell when it rains?
— Chris St. John, Fort Myers, Florida

It's not rain you're smelling; it's the earth. The piquant, musky odor that hangs in the air emanates from an odorous chemical buried in the soil called geosmin (literally, "earth smell"). One way to detect that "rain" scent is to unearth the geosmin yourself by rooting around in your garden. But a good rain shower will do the trick as well. As a storm moves in, the atmospheric pressure drops and equalizes with the pressure in the ground, causing the earth to "outgas" geosmin. "When the barometric pressure drops, the soil exhales," explains Mary Firestone, a soil microbiologist at the University of California, Berkeley. Because odors transmit more efficiently in wet air, a mere hint of geosmin is enough to get our olfactories primed for a shower.

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