 Some people wear T-shirts with Question Authority! written on them. Questioning authority is generally regarded as a matter of individual discretion. The surprise test paradox shows that it is sometimes mandatory. The student is rationally required to doubt the teacher’s announcement even though the teacher has not given any new evidence of being unreliable. For when only one day remains, the announcement entails (iii), a statement is that is impossible for the student to know. The student can foresee that this forced loss of knowledge opens an opportunity for the teacher to give the surprise test. This foreknowledge is available at the time the announcement.