By Patrick Clinton

What is quicksand? Where does it occur? And is it as dangerous as in the movies?
Christian Timmerman, Phoenix, Arizona

Quicksand can be found anywhere on earth, wherever the conditions are right. It's a kind of thick soup created when water flows up through sand. It can develop, for example, along the inland side of a levee when a river is in flood stage or when an underground spring flows beneath a silty bank. The flow can cancel out the weight of the sand. Instead of packing together like a solid, the sand granules float around. When you step into it, there's nothing to support your weight, and you start to sink. You won't sink far, though. Quicksand is thicker than water, so in the long run you'll float. You probably could drown if you put your mind to it, but your corpse wouldn't go anywhere.

If you should step in quicksand: Get on your back and let your feet float to the surface. Then squidge slowly to stable land. It'll take time. I've heard of people taking all day to move a half-dozen feet in quicksand. You could starve, but you won't sink.

On the other hand, animals that can't float on their backs-elephants, for instance-are in real danger of drowning in quicksand. In fact, some researchers believe that the legendary "elephant graveyards" might actually be patches of former bog or quicksand where groups of pachyderms got trapped. In any event, you can ignore those old Tarzan movies: Quicksand can't suck you in. Hollywood's another matter.

Why does water come in so many shades of blue?
Paul Hohenleitner, Langhorne, Pennsylvania

"Water transmits and scatters blue better than any other color," notes physicist Howard R. Gordon of the University of Miami Coral Gables. Translation: As it passes deeper into the ocean, sunlight grows progressively bluer as other colors in the spectrum are absorbed. In general, deep, clear water is going to look the bluest, especially under a bright blue sky. If you have shallow water over a sandy bottom, it'll reflect white and green light, giving you an azure color. In overcast weather, the sea will turn a slate blue, the result of the reflection of clouds on the surface. Finally, phytoplankton suspended in water will give it a greenish-blue hue.

Where is the bluest ocean water, in the opinion of a professional? The Atlantic's Sargasso Sea. "It has the richest blue on the planet," Gordon says, "with less pollution and less phytoplankton than anywhere. And it's about four miles deep."

Why do pigeons constantly bob their heads?
Ben Kaplan, Burlington, Vermont

Head-bobbing is a common trait among ground birds like pigeons, pheasants, partridges, and chickens. Ornithologists call it the "optokinetic response," and it seems to help the birds' vision. Remember that a pigeon's eyes are set on the sides of its head, so that when it's walking around, the world sort of sails by in a confusing blur, like landscape viewed from a moving train.

The optokinetic response appears to compensate for this. Next time you're down at St. Mark's Plaza, take a look: The pigeon has a kind of inchworm gait. It jerks its head forward, then brings its body to meet it, then jerks its head forward again. The net result: The bird gets a series of fixed snapshot images, rather than a long, continuous blurry one. (A twirling ballerina uses a similar strategy, keeping her head aimed at a fixed point while her body rotates.)

Back in the late seventies, Barrie J. Frost, a visual neuroscientist at Queens University in Kingston, Ontario, put pigeons on a tiny treadmill. They walked, but they didn't move relative to their environment-and they stopped bobbing. Draw your own conclusions; to me it's scientific proof that if you're going to get anywhere, you have to stick your neck out.

See Also:
Stargazing

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