If you think house hunting is hard, consider the plight of this snail. It lives only in tide pools in southern Japan. Within those tide pools, it only lives in holes carved out of rock—specifically, holes dug by sea urchins. But it can only move into one of those holes after the hole-digging urchin has moved out. When a second, differently shaped sea urchin moves into the hole, it leaves a gap between its spiny body and the wall of the burrow. It's this nook that the snail snuggles into.