For candidates who want to prepare for every possible question that could be thrown at them, the format for Tuesday night's presidential debate is slightly terrifying: It is a town hall-style debate, which means that the questions come from uncommitted voters. When political reporters are posing the questions, the candidates usually have a good idea what to expect. But when members of the public get the chance to weigh in, the candidates can sometimes face curve balls that leave them flummoxed.