The third thing in the interview is the impossible question. This is fun. The idea is to ask a question that they have no possible way of answering, just to see how they handle it.
"How many optometrists are there in Seattle?"
"How many tons does the Washington Monument weigh?"
"How many gas stations are in Los Angeles?"
"How many piano tuners are there in New York?"
Smart candidates will realize that you are not quizzing them on their knowledge, and they will enthusiastically leap into trying to figure out some back-of-the-envelope answer. "Well, lets see, the population of LA is about 7 million; each person in LA has about 2.5 cars..." Of course it's OK if they are radically wrong. The important thing is that they leap into the question enthusiastically.
They may try to figure out the capacity of a gas station. "Gee, it takes 4 minutes to tank up, gas stations have about 10 pumps and are open about 18 hours a day..." They may try to figure it out by area. Sometimes they will surprise you with their creativity or ask for a Los Angeles yellow pages. All good signs.
Not-so-smart candidates will get flustered and upset. They will just stare at you like you landed from Mars. You have to coach them. "Well, if you were building a new city the size of Los Angeles, how many gas stations would you put in it?" You can give them little hints. "How long does it take to fill up a tank of gas?"
With not-smart candidates, you will have to drag them along while they sit there stupidly and wait for you to rescue them. These people are not problem solvers and we don't want them working for us.
Joel On Interviewing 10: The Impossible Question