When you call up to apply, you get asked a relatively easy multiple-choice question. If you get that right, your details get noted down. One hundred people who answered the question correctly get called up and asked a question to which the answer is usually something like a large number (so that people are effectively trying to get the closest they can to the correct answer, rather than just knowing a fact, eg how many miles is it from Earth to Pluto). The ten people who get closest to the correct answer are the ten who will appear on the show. Hope that makes sense; it's a little garbled . . .