Purpose: To get students thinking and talking about math processes.
Students often are so obsessed with finding the answer to a question that they don't think (or care) about how they arrived at the answer. To discourage this, do activities where you "devalue" the answer by providing it to the student and then challenging them to prove it is correct.
For example, rather than asking what is 10 + 20?, ask them to prove that 10+20 = 30.
You can also reword the problem..
Change it from: