This lesson focuses on understanding the meaning of subtraction as it relates to addition. Students solve "add to with change unknown" problems (for example, "Once upon a time, 4 little bears went to play tag in the forest. Some more bears came over. In the end, there were 6 little bears playing tag in the woods. How many more bears came to play?"). Rather than thinking about these situations only in terms of addition, as in previous lessons (for example, 4 + ___ = 6), they now think in terms of subtraction (6 - 4 = ___) as well. By writing two number sentences (one with addition and one with subtraction) to represent each problem situation, students come to understand subtraction as an "unknown addend" problem.