Feisty Squirrel Protects Jewelry Store Cash Register

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This security squirrel's job is nuts and that's just the way he likes it.

You'd have to be nuts not to

from here and here (image source)

Of course with the chain on the outside the squirrels aren't actually keeping anybody out, but I've seen humans make equivalent blunders.

They'll never be able to break into here

found on Good Bear Comics

It might be a bit on the subtle side, unless you've spent time looking into squirrel-proof bird feeders. That little bit below the birdhouse is a barrier that squirrels are supposedly unable to get past (though you may find that some are acrobatic enough to find a way). Check the comic on it's original site, though - there's a bonus panel that explains how the bird intends to deal with those acrobat squirrels.