QUESTION: Why is it that when you have something floating in your beverage and you turn the glass the floating thing doesn't budge?

ANSWER: This is a matter of fluid dynamics, and the first thing you need to know is that a fluid is not the same thing as a liquid. A liquid is a fluid. But a gas is also a fluid. Honest! A fluid is simply something that's not a solid. A solid is something in which the molecules are locked into a lattice. Someday, we'll figure out what the heck "plasma" is.So there you are, at a cocktail party, talking to the host, and you suddenly realize that in your glass contains a floater of unidentifiable composition. Naturally, it is poised to enter your mouth the moment you take a sip, so you pivot the glass, hoping the floater will travel 180 degrees to the other side. It refuses. You have no choice but to fish it out with your finger, and seconds later are forced to shake hands with someone very important who immediately grimaces at the sensation of your wet hand. You say, "Sorry, I was just retrieving a floater from my chablis." It's a total nightmare.

Your mistake was a lack of patience. If you had waited long enough, the floater would have moved. When you spin the glass, you create "sheer" in the liquid. The liquid touching the glass's surface moves as you move the glass; it does not "slip" because it's held in place by friction. But the adjacent layers of liquid slip more and more the further you go from the edge of the glass. Eventually all the liquid will rotate and eliminate the sheer. The speed with which this happens varies with the viscosity of the liquid. A thick liquid has less sheer than a thin one.

Which means a Bloody Mary turns better than white wine.

QUESTION: Why do we do things by the "rule of thumb"?

ANSWER: We humans should always remember that we are a thumb-based species. Have you ever looked closely at your thumb? The first thing you'll notice is that it's way down on the SIDE of your hand. It emerges practically from your wrist. It's fat and stocky, and is missing a joint. But it's still very handy - hah! - even if you can't pick your nose with it unless you have giant nostrils.

Now that the "comedy" portion of this item is out of the way, let's get to the answer. A fellow named Chris infiltrated the Why bunker recently and told us that "rule of thumb" dates back to Olden Times, when it referred to the etiquette of a man beating his wife. Supposedly the rule was that a man couldn't beat his wife with anything larger than a thumb. (It's not clear whether this rules out using the hand itself. Maybe the guy could only use his thumb. The wife would no doubt shriek, "Stop thumbing me!")

Naturally, we immediately checked to see if this ghastly explanation is in our reference books. It is not. "The Dictionary of Word and Phrase Origins" by Adrian Room says there are two possible origins. The thumb may have been a crude measuring device, with the lower half roughly equal to an inch. More likely, says the dictionary, the phrase refers to the practice of brewers using their thumbs to measure the temperature of their concoctions.

The Mailbag:

S.A. Hermenet of Penfield, N.Y., asks, "If I were to drill a hole through the center of the earth (pole to pole) and then dropped a baseball in the hole, would the ball stop at the center?"

This is such a good question we'll pretend it's a Why question. Later we'll deny we fudged with our rigid format.

It is possible to imagine an incredible message delivery system using holes through the earth. But we'd have to cheat a little. We'd have to have a magically frictionless tube. And it would have to be a perfect vacuum. Nothing could rob any energy from our plummeting baseball.

So this is what happens. You drop the ball in the hole, and 84 minutes later the ball reappears right back at your feet. Then, it's gone again, for another 84 minutes. This is the Schuler Period, named after a certain Max Schuler.

See, the moment you drop the ball it starts accelerating toward the center of the earth. It hits maximum velocity in the center, about 18,000 miles an hour, according to Chuck Counselman of M.I.T. The ball then slows down until it finally stops moving at the other end of the earth. Then it starts falling again, right back to you.