You know those little, annoying, seed "key" things that blow around the yard? Yeah, well if you don't rake them up, they can turn into a stump.
Actually, they can grow into a tree which wasn't properly disposed of. Said tree can continue to sprout branches, no matter what you might do to discourage the growth.
Sam has started to dig at this stump. He's already dug out a big chunk of dirt on the one side, I think if we leave him to it, he may dig up enough to kill the tree. That would not hurt my feelings one little bit.
Sam digs digging. I do wonder though, what he's done with the dirt he's removed.