Through discussion, teachers help students understand the role of nonexamples as well as examples in informal proof, as demonstrated in a study of young students (Carpenter and Levi 1999, p. 8). The students seemed to understand that number sentences like 0 + 5869 = 5869 were always true. The teacher asked them to state a rule. Ann said, "Anything with a zero can be the right answer." Mike offered a counterexample: "No. Because if it was 100 + 100 that's 200." Ann understood that this invalidated her rule, so she rephrased it, "I said, umm, if you have a zero in it, it can't be like 100, because you want just plain zero like 0 + 7 = 7.