QUESTION: Why does the world turn?

ANSWER: Today's key term: the Conservation of Angular Momentum. Repeat as necessary.Long ago, before the Earth formed, there was just a big dust cloud hovering in space. It wasn't perfectly round. There's too much randomness in the cosmos. In fact it might have looked like a chicken or a horse - you know how clouds are.

There's one other random quality: The cloud was turning slightly. Not much. Just a little. Theoretically a dust cloud can be static, but "that's incredibly unlikely given the rather turbulent nature of the interstellar medium," says Steven Stahler, professor of physics at the Massachusetts Institute of Technology.

So there's this slight spin. But by the time the dust cloud contracts and forms a planet, it's spinning much faster. Why does it speed up? A good analogy is a figure skater: She goes into a spin with her arms outstretched, and when she pulls them in the rate of spin rapidly increases. This is the conservation of momentum. The dust cloud, likewise, contracted perhaps a millionfold, condensing into the solid Earth, and to "conserve" the initially slight spin of the huge cloud, the Earth had to spin rapidly.

The bad news is, the Earth is slowing down. This is largely because of tides. Twice a day, around the world, ocean water surges in and out of bays and lagoons and river deltas, following the gravitational tug of the moon. The energy to move that water has to come from somewhere. One place is the Earth. The tides steal some of the energy of the Earth's spin. Eventually the Earth day will be as long as a lunar month; half the Earth will face the moon and the other half will never see it.

Then the moon's orbit will start to get smaller, until the moon gets so close to the Earth that it is ripped apart by the Earth's gravitational field. For a while the Earth may have rings, like Saturn, but eventually the moon fragments go you-know-where.

"It will come raining down on our heads, or the heads of whoever is around at the time," says Chuck Counselman, professor of planetary sciences at MIT. "It would be a major ecological catastrophe, yes."

How should you, the reader, react to this news? First, by reminding yourself that this column is not on the front page of the newspaper. When moon-raining-down stories make A1, THEN you start worrying. The fact is, before the moon breaks up the sun may enter its terminal stage in which it will become unstable, expand dramatically and incinerate the Earth before collapsing into a white dwarf star. So cheer up.

QUESTION: Why is a dollar sometimes called a "simoleon"?

ANSWER: This is crusty, old-guy slang, along the lines of "sawbuck" and "dame." A young, hip person nowadays refers to dollars as "bones," as in "I'm jonesing for those killer spikes, but they cost 125 bones." (Translation: The nice shoes I very much desire are too expensive.)

"Simoleon" is pretty mysterious. The earliest known usage was in a George Ade story in 1896: "He said I could have it for 400 samoleons." The spelling "simoleon" soon became standard. The Oxford English Dictionary says, lamely, that it might be derived from "Napoleon." That's it. End of explanation.

Fortunately, we got emergency word service from David Jost, senior lexicographer at the American Heritage Dictionary, and he filled in some more of the background. A "simon" was, prior to about 1700, a slang term in England for a sixpence, though it's not clear who the Simon was that inspired the slang. The term emigrated to the United States and by the mid-1800s was slang for a dollar. At the same time, the French were using a 20-franc gold coin called a Napoleon. Someone (by the time of George Ade) managed to combine simon with Napoleon to get simoleon. That's the best we can do at the moment.

Frankly we prefer the contemporary slang of today's younger generation. We're perusing "Slang U: The Official Dictionary of College Slang," by Pamela Munro, and note such charming terms as "eye booger" (so much for "sleepy sand"); "buttly," an adjective for someone who's unattractive; "squid," a new name for a jerk or a nerd; and "zuke," one of 31 listed terms for throwing up (it has, you must confess, an economy that does not grace the synonymous "talk to Ralph on the big white phone").

The book's in paperback, by the way, for a mere ten simoleons - uh, bones.

The Mailbag:

Frank A. of Bethesda, Md., responds to our recent statement that nursery rhymes are incredibly brutal: "I thought you might be interested in knowing that `Ring Around a Rosey' got its origins from the Black Plague. . . . refers to the sore on the skin that was the first sign of affliction - a rose-colored spot surrounded by a similarly colored ring. `Pocket full of posies' refers to the fact that (still living) victims often stuffed their clothing full of flowers to mask the stench of their rotting flesh."

How lovely. Kind of makes you want to zuke.