Bird feeders are supposed to be for birds; but apparently nobody told the squirrels that!  I've seen a lot of complex methods for keeping our furry "friends" away from the bird seed (a motion-sensor attached to a garden hose is probably my favorite); but this solution might be a little cheaper and easier.  Actually, it definitely is.

It's another classic case of taking one thing and finding a new use for it.  Just don't tell your kids you were going through their toy box!