If I were stranded in the desert and needed water, could I extract it from a cactus?

—Jackson Adams, Atlanta, Georgia

Those classic western scenes, in which movie-star cowboys slash open saguaros and slurp fresh water, are pure Hollywood. Out of some 1,800 known kinds of cactus, only the seven-foot-tall Echinocactus and Ferocactus barrel cacti, found in Mexico and the desert Southwest, are equipped with reservoirs large enough to stave off dehydration. Imbibing, however, would not be a pleasant experience: Beneath their waxy skin is a spongy tissue laced with bitter, slimy alkaloids that have the toxicity of bug spray—all part of the succulents' cunning defense against desert scavengers. (Native Americans extract water for their hardy livestock by using a rock to pound moisture from the pulpy innards.) While a large swig of this juice could kill a field mouse, human pilferers would likely suffer vomiting and diarrhea—both of which only exacerbate dehydration. "In a life-or-death situation," advises horticulturist Mark Sitter of the Arizona-Sonora Desert Museum, "you'd be better off drinking your own urine."

How do fish sleep?

—E. Hagerman, Santa Fe, New Mexico

The mystery of fish slumber lies within the greater mystery of the piscine mind. If you've ever cast for a fish only to be outsmarted, you'll probably be discouraged to learn that a trout's brain is about the size of an almond. This tiny bundle of nerves lacks a cortex, the tissue that in more complex animals, like mammals, generates electric impulses. By monitoring these brain waves, scientists have learned that humans experience four distinct stages of sleep during a typical 90-minute period, that dolphins can swim and snooze at the same time, and that horses can doze while standing. Scientists, however, can't so easily decipher the electrical impulses of the piscine brain. And the other helpful clue—closed eyes—is worthless, since fish don't have eyelids. Even so, behavioral studies suggest that they must sleep to survive, and that they go about it in pretty much the same way as the rest of us: by slowing their heart rates and becoming oblivious to mild annoyances. Biologists have noted that catfish remain motionless during the day and perk up to feed at night; sticklebacks, on the other hand, appear to rest for shorter periods, while tropical parrot fish tuck themselves into coral crevices at night and don't emerge until morning. If not technically "sleeping," these fish are at least satisfying sleep's key evolutionary functions: to conserve energy and recharge for another day of outwitting predators like you.

What causes undertow, and does it create better surf breaks?

—J. Axton, Malibu, California

Lifeguards call it "run out" and oceanographers refer to it as "rip current." No matter what the moniker, if you've ever been pummeled head-first into the beach and scraped along the sand, you'll probably concur that undertow—the strong bottom-current that returns water from the shoreline to the sea—can be terrifying. Here's how it works: An endless succession of breaking waves deposits water onto the beach, where it piles up, held there by the incoming breakers. As soon as a low spot in an advancing roller offers an opening, however, the trappedwater rushes under it, restoring hydraulic balance and creating a formidable ocean gush of up to four knots. As the incoming waves crest over the undertow, they flatten and break quickly—not exactly prime hang-ten conditions."Surfers should look for waves that move along a shoreline instead of collapsing all at once," advises Chris Sherwood, a coastal geologist with the U.S. Geological Survey. "Killer undertow is not what you want."

What determines tree line?

—Michael Edgar, Cambridge, Massachusetts

Tree line is one of those slippery topics dependent on dozens of variables, the most important of which is temperature. As pines, spruce, and fir get higher in elevation or farther north in latitude, they respond to an increasingly chilly environment by growing shorter and farther apart. At a certain point, they stop germinating altogether. Dendrologists call this line the ten-degree isotherm: Trees generally don't grow where the mean temperature for the month of July is less than ten degrees Celsius (50 degrees Fahrenheit). But there's topography, soil type, wind, precipitation, and sunshine to consider as well. In the White Mountains of New Hampshire, for example, tree line falls at roughly 5,200 feet on sunnier, protected, south-facing slopes, but at only 4,800 feet on the penumbral northern flanks. Coastal ranges like the Cascades, where summers are relatively cool and cloudy, bear trees up to 6,000 feet, while in the warmer, clearer Rocky Mountains, pines flourish well above 10,000 feet. The ten-degree rule may seem maddeningly imprecise, but to arrive at a more rigorous formula, says Graeme Berlyn of the Yale School of Forestry, "you'd have to start analyzing leaf temperature." Be our guest.

Illustrations: Jason Schneider

Send your questions for The Wild File to Outside, 400 Market St., Santa Fe, NM 87501, or submit them here.