If you want to get your fingers in the pi tree, you're going to have to look for it. This isn't a tree made of pie, or growing fully-baked pies. It's a pi tree, one that exists infinitely. It's the most rare tree in the world. And if you want to stick your fingers in it, you have to search for it, far and wide!
You try to look for the pi tree for so long that you can't help but get a very deep tan (たん). You can't search for anything inside, and you definitely can't find the pi tree inside, so you spend a lot of time outside in the bright sun.