There are few things that can make you feel truly dumb better than a clever optical illusion.
There's something about them that is so awfully addictive. You can't help but feel yourself inexplicably drawn to them, nearly banging your head against the table in an attempt to figure them out. It's incredibly frustrating.
But when you finally do arrive at the answer, it feels like a great weight's been lifted, and you can't help but pat yourself on the back for figuring out a uselessly fun riddle.