The squirrel loves a particular tree and is happiest eating pinecones in its shade. But then the squirrel starts to worry. What if someone else decides it's THEIR tree? What if that someone wants to eat THEIR pinecones in the shade of THEIR tree? Should the squirrel build a gate? Or maybe a wall? Yes, a wall. The squirrel will build a long and high wall that no one can get over or around. Only, now that there's a wall, how can the squirrel know what's on the other side of it? Maybe a better tree is out there, full of pinecones. Maybe even a whole forest of better trees...