The authors describe a lesson--"You Decide"--which challenges students but also provides opportunities for success for those who may struggle. They show how this lesson has been helpful for teachers in revealing some misconceptions that often exist in primary students' thinking. In this article, they share data on the apparent relative difficulty of the items for Year 5/6 students, and possible reasons for this. One outcome of this activity is likely to be that learners come to realise that many problems lend themselves to being easily solved through mental methods if the meaning of the number sizes, expression, and operation are understood.