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Question: Since we're mostly made of water, why don't we slosh around? You always hear that our bodies are three-fourths water, or something like that. So why aren't we sloshier? Shouldn't we just spread all over the floor? Why don't football players, when they collide, make a splash?

Answer: The first thing you have to realize is that some people are more watery than others. Lean, muscular people have more water in them than fat people, because there's lots of water in muscles and not much in fatty tissues (fat and water don't mix).And our water content is in constant flux. You lose about a pint of water a day just exhaling, the air carrying away water from your lungs. You can sweat away as much as 10 pints a day.

Your water content also varies with age. A fetus is about 90 percent water, a newborn about 70 percent water and a mature adult about 60 percent water, according to the Federation of American Societies for Experimental Biology.

The fact is, we're mostly made of oxygen atoms. Our handy biology textbook says 65 percent of a human being is composed of the element oxygen. Another 17.5 per-cent is carbon, 10.2 percent hydrogen, with some calcium and nitrogen and phosphorous making up most of the rest. (Some of us have lead in certain places).

Jeffrey Rosenstein, a GWU anatomist, says, "We're not a water, we're a gel."

This is because a lot of the water in our bodies combines with other elements to form complex compounds, such as proteins, carbohydrates and lipids. (The Why staff is feeling particularly lipid lately.)

Individual cells are like gelcaps. The goop is contained. Between the cells is more goop. The larger goopy areas are kept in place by, among other things, firm collagen tissues, which in turn are held in place by the skeleton. "The reason we don't slop apart is that all these cells are attached to your bony framework," says Rosenstein.

So now you see why we have bones. Otherwise we'd quiver and tremble like Jell-O.

Question: Why have numerous cultures come up with the silly idea of mermaids?

Answer: Sea cows. No lie. The dugong, or sea cow, which is related to the manatee, nurses its young in an upright posture. The mother's teats resemble human breasts. The babies are held against the mother with a single fin, like a human holding a baby with one arm.

The problem is that it is hard to imagine that sailors would look overboard, see some large, blubbery, hairless mammal, and find themselves irresistibly attracted. You'd have to be at sea a long time to get to that point.

Richard Grinker, an anthropologist at George Washington University, says that mermaids are part of a broader phenomenon, the concept of the half-human, half-beast. People have always believed themselves to be separate from nature. Half-human creatures are pre-Darwinian transitional forms. They fill a perceived gap in nature - they're like missing links.

You can't ignore the fact that there's a slap at women in the myth of the mermaid. The mermaid is always alluring, sitting on the rocks combing her hair and looking in a little mirror, but she's also exceedingly dangerous, deceptive, ultimately elusive. This is the basic male take on women.

"That age-old idea of the gorgeous and deadly woman has pow-ered much myth and art. Mermaids crystallize the fear," writes Diane Ackerman in her new book "A Natural History of Love."

"For men of the sea, mermaids combine the self-destructiveness of the ocean, to which they are nonetheless wedded, with their loneliness for the women they've left behind," she writes.

Go too close to them and your ship crashes on the shoals. Odysseus had to lash himself to the mast to keep himself from being lured to his doom by the song of the sirens. (We understand that particular strait in the Dardanelles now plays the Muzak version.)

The Mailbag:

John P. of Kissimmee, Fla., writes, "The spot which I occupy is traveling at about 1,000 mph as the Earth rotates around its axis, an the entire planet is moving at about 67,000 mph in its orbit around the sun. Since my speed in relation to the Sun varies by about 2,000 mph from the midpoint of `day' to the midpoint of `night,' why don't I feel the change of velocity as I do on a carnival ride which features movement around two axes?"

Dear John: You are thinking too much. Gear down a notch.

When you go on a carnival ride you endure sudden and severe ac-cel-era-tions through space. The motions of Earth - both its spin and its movement through the orbital plane - are gentle by contrast.

You are right to wonder if there are slight accelerations and decelerations that affect you. The planet is always accelerating toward the sun, and you, personally, accelerate faster when you are closer to the sun than when you are farther away, because gravity gets weaker with distance.

But these are tiny, tiny factors compared with the Earth's own gravitational field. And even Earth's gravity isn't so terribly strong - that's why you can jump in the air. (Not to imply that anyone on the Why staff can dunk a basketball.)