Now that's one question you've probably never asked anyone in a job interview (or anywhere else). But how an applicant answers it could reveal more about future performance than the usual inquiries about previous positions, accomplishments, goals, and the like.

At least that's the thinking at Microsoft, where hundreds of job seekers have been asked the bathroom question as part of the legendary "interview loop" -- a rigorous ritual in which candidates are grilled by their future colleagues with a barrage of puzzles, riddles, and bizarre hypothetical questions. The process has been one of Microsoft's closely guarded secrets. But science writer William Poundstone sheds light on it in his new book, How Would You Move Mount Fuji? Microsoft's Cult of the Puzzle: How the World's Smartest Companies Select the Most Creative Thinkers.

Companies like Microsoft, Poundstone says, are less interested in what applicants have done or know than in how they think. Take the matter of Mr. Gates's commode. Applicants are supposed to come up with ideas Gates hasn't thought of himself -- no small feat, considering that Gates is able to fill his bathtub to desired temperature via remote control from his car. Some winning answers: Medicine cabinets that automatically lock to keep out children; a mirror that doesn't reverse left or right, but rather, shows your own image the way other people would see you. "It's vital to hire people who can not only deal with change but initiate it," Poundstone says. "Microsoft needs to hire people capable of inventing the Microsoft of five or 10 years hence."

Here are a few examples from Poundstone's book on Microsoft's rigorous interview process:

How do they make M&Ms?

Here's the "correct" answer: The chocolate centers of "plain" M&Ms are cast in little molds. The chocolate ellipsoids are then put in a big rotating drum. While jostling in the drum, they are sprayed with a sugary liquid that hardens into a white candy shell. The candies are then squirted with a second, colored sugar liquid. But to Microsoft, getting the right answer isn't important. It's the ingenuity of the proposed solution, and the process that you use to arrive at it, that's evaluated. One ingenious, if wrong, answer: "There's a sheet of hot, boiling chocolate, and they freeze the peanuts and fire them through it so it instantly freezes and the chocolate is hard by the time it hits the ground."

How would you weigh an airplane without a scale?

This question attempts to measure an applicant's quotient for logical thinking. One answer: Taxi or fly the jet onto a ship big enough to hold it. Paint a mark on the hull of the ship showing the water level. Then remove the jet. Now, load the ship with items of known weight (say, 100-pound bales of cotton) until it sinks to exactly the line you painted on the hull. The total weight of the items will equal the weight of the jet.

How would you test a saltshaker?

Microsoft asks lots of testing questions, to determine whether candidates are aware that multiple criteria apply in evaluating even the simplest artifacts. The best candidates, the company has found, question assumptions and see things from novel perspectives. What could go wrong with, say, a saltshaker? A saltshaker can be filled with sugar; the holes could be the wrong size; it's hard to tell the saltshaker from the peppershaker; it's too hard to refill. Once you outline several levels of criteria, you could have a focus group test a variety of shakers in different realistic situations to see which designs work best.

Most companies probably don't need to be as ruthless as Microsoft is about hiring what it calls the "the top 10% of the top 10%." On the other hand, having a few Mensa members on staff certainly has its appeal.

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