When the Great Flood subsided and the Ark came to rest, Noah let the animals out with the admonition to go forth and multiply.<p>Some time later, Noah went for a walk to check on the animals and their new families. All was well until he reached a pair of snakes, who had no children. Worried, he asked them what was wrong. "Please," the snakes said, "Cut down some trees, that we might make our home in them." Noah was confused, but did as they requested.<p>Next time he passed by, Noah was delighted to find a happy family with many young snakes. Noah congratulated them, but was still confused, and asked why the felled trees were so important.<p>"We're adders, you see," the snakes replied, "So we need logs to multiply."<p>Edit: Argh, I somehow managed to completely overlook that this joke was listed in the runner-ups in the linked article. Sigh. I'll leave it here anyway because it's my favorite math joke.