Has everything to do with muscle control. It's on the same precept as being able to do the "Spock" hand thing or flipping your tongue over. It's something you can learn to do with practice, at least I managed to be able to spread my fingers in any combination on either hand though when I first tried I couldn't do it at all. Couldn't wink either, though I've learned to do it with both eyes easily now.
Still can't for the life of me get one eyebrow to raise without the other though. Been trying for years, don't think I'll ever get it. Can't whistle either lol
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I am Jon Stewart with some Colbert cynicism, Thomas Edison's curiousity, wrapped around a hardcore gamer sprinkled very liberally with Deadpool, and finished off with an almost Poison Ivy-esque love/hate relationship with humanity flourish.