By the end of second grade, students also should use properties to reason about numbers. For example, a teacher might ask, "Which number does not belong and why: 3, 12, 16, 30?" Confronted with this question, a student might argue that 3 does not belong because it is the only single-digit number or is the only odd number. Another student might say that 16 does not belong because "you do not say it when counting by threes." A third student might have yet another idea and state that 30 is the only number "you say when counting by tens.