While listening to the birds sing in the woods, the old carpenter was remembering what it was like when he was a child. Back then, the woods were home to many birds, but pollution and the construction of new buildings forced them out of their homes. He decided to build nests for the birds. First, he built two nests. Then a pair of titmouse and a pair of eastern gray wagtail flew in. Two pairs of birds in two separate nests, 2×2 = 4. That makes four. The carpenter was delighted and started building more nests.