A bunch of friends visits a young guy who recently moved to Las Vegas. "Here's how we get twenty bucks in Las Vegas," he boasts to his friends, putting $20 on the roulette wheel for black. When it comes up red, he puts down a $40 bet, bragging that "I'll keep increasing the size of my bet until I make back my money -- plus another $20!"

One time he'd had to double his bet four times in a row, but he's convinced that his system works, and does it every time company comes to visit.

Do you see where this is going? One day he hits a horrible streak. Five times in a row he's lost the bet. (So, $20, $40, $80, $160, $320.) Now he's got to bet another $640 -- and hope that he wins. (It's getting awkward, with all his friends watching him lose, feeling bad for him...) At some point his wallet is out of cash, and he's slinking back to the in-casino ATM machine. (And the bank balance isn't infinite either...)

Conclusion? This particular strategy has an asymmetric downside. More often than not, you'll walk away with $20. But the casino knows that sooner or later you'll have that one very bad day where they'll get it all back.