In January you wrote that strong undertow coincides with mediocre surfing. So what constitutes the perfect wave?

—Bill Donovan, Chicago, Illinois

"Once you ride a really great wave, you can't help but think a better one exists," says Patrick Caldwell of the National Oceanographic Data Center in Hawaii. The wave of surfer fantasies is a plunging breaker with a cylindrical shape, a tube that carries the rider hundreds of yards, allowing a clean escape before it finally collapses. The tidal effect is tricky--a rising tide creates the best surf in some spots; in others, falling tide is what you want. A good wind from land to sea helps create smooth, surfable surfaces. But nothing affects a wave more than the coastal sea floor: A steep seaward-facing slope of sand, rock, or coral produces the largest swells. Here the wave's bottom encounters shallower water and slows, while the top of the wave, unchecked by drag from the ocean floor, pitches and curls at its original speed. To yield a surfable tube, the incoming ocean current must also approach the reef at a sharp angle (30 to 60 degrees is optimum). This keeps the wave peeling for a considerable distance. Such are the forces at work on Oahu's massive Banzai Pipeline, and the stuff of endless summer.

Do any animals (other than humans) laugh?

—Andrea Reisman, Sarasota, Florida

Get three zoologists in a room to discuss this question and the debate will inevitably turn philosophical, divided among the empirical camp ("if it sounds like a laugh, call it a laugh"), the pragmatic ("if it has the same purpose as a laugh, call it a laugh"), and the antianthropomorphic ("it's an animal, so don't even think of calling it a laugh"). The experts do agree that laughter is an instinctual response dating at least as far back as six million years ago, sometime after our ancestors, Homo erectus, began walking upright. They are also careful to distinguish the chuckle of a tickled baby from the reaction to a Chris Rock routine; the cognitive ability to get a joke doesn't appear in humans until age six. Researchers who believe that animals laugh mean the tickled-baby variety. Witness the chimpanzee: Find its ticklish spots and it emits a laughlike vocalization and forms an expression that looks very much like a smile. Elsewhere on the evolutionary tree, Jaak Panksepp, a neuroscientist at Bowling Green State University, has found that tickled lab rats make chirping sounds that he insists are homologues to the laughter of human babies. Panksepp has encountered some skepticism. The behaviorist camp, he says, "would prefer to castrate me rather than believe that rats laugh."

If you get caught in the rain, which will keep you drier--running or walking?

—Morgan King, Unity, Maine

Our instinctive sprint toward shelter really is the best way to avoid getting soaked, according to Trevor Wallis and Tom Peterson, meteorologists at the National Climatic Data Center in Asheville, North Carolina. A runner, after all, spends less time getting pelted by raindrops. But this theory has not gone unchallenged. In 1995, four British meteorologists used a mathematical model to argue that the speed difference between a runner and a walker is too small to make a difference in a storm; each, they said, gets equally soaked. Wallis and Peterson, both Type A joggers, disagreed and donned identical cotton sweatsuits and caps and stepped out into a downpour. Peterson walked a 100-meter track at a three-mile-per-hour pace, while Wallis ran it at nine miles per hour. Inside, the two weighed their clothing and found that Peterson's had absorbed half a pound of water, compared to Wallis's one-third of a pound. The Americans concluded that a walker will get 30 to 50 percent wetter than a runner. Calculations aside, there's one sure way to stay dry. Says Peterson, "I'd recommend an umbrella."

Cumulatively, how many people have lived on Earth?

—Alex Ciancaglini, Denver, Colorado

The best answer may be off by several billion due to two unsolved mysteries: The date the first humans are thought to have appeared varies by 50,000 years (from 75,000 b.c. to 25,000 b.c.); and scientists have no way to document every plague, war, and climate change that has caused our numbers to ebb and flow. Still, the question makes for a decent college-level math problem. Between the appearance of our subspecies Homo sapiens, or modern humans, and the dawn of agriculture (8,000 b.c.), the world's population grew to five million. By a.d. 1, our ranks had increased to 300 million. Though the Black Death--the bubonic plague--wiped out three-quarters of Europe and Asia in the 14th century, by 1800 our species numbered one billion. Last November, the human population reached six billion. Crunch the numbers using the growth rates for each era and you've got 106 billion humans, more than 5 percent of whom are alive today. Think that's a lot? Compare our numbers to ants, whose present population, according to the esteemed sociobiologist E. O. Wilson, is in the neighborhood of a quadrillion.   

Illustrations: Brian Rea

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