According to author Dylan Wiliam, because lessons never go exactly as planned, teachers should build plan B into plan A. This involves designing a lesson with a "hinge" somewhere in the middle and using specific kinds of questions--what he calls hinge questions--to quickly assess students' understanding of a concept before moving on. Wiliam offers several principles for designing good hinge questions. He suggests how teachers can get an on-the-spot response from each and every student in the classroom, quickly assess student understanding in view of either moving forward with the lesson or going back to review, and help ensure through proper question formulation that students don't get the correct answer for the wrong reason. By planning at least a few questions carefully, teachers can get better-quality evidence about what their students can and can't do in time to do something about it.