Squirrels are really smart. I once tried to chase one away from a tree using a fishing rod, but it just moved to a distance an inch longer than where the rod could reach and stayed put, just keeping an eye on me while I was there.

They are known to spy on each other and to steal each other's "investments." (Also, if one notices that another is watching, it will pretend like it's hiding the food here and then take it someplace else.)