Your friend orders a bottle of beer, but you realize there's something horribly wrong with it: You're not the one holding it. So, you propose a deal: You ask your friend to find the bottle's bar code and read you all the digits except the last, and if you can guess what the last digit is, the sweet elixir is yours for free. "Aha!" your skeptical drunken friend will say. "I bet every bottle of beer has the same digit at the end!" Nope. Tell him he can grab any package with a bar code, or just go around and look at them -- they're all different, even among the same products. So your friend says yes, obviously, because you only have a 1 in 10 chance of picking the right number. Then you do exactly that, and he's so shocked that he empties the bottle on your face. Hey, free beer is free beer.
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Tossing the drink may count as a bar code violation.
The last digit of a bar code, or the check digit, is pretty easy to figure out because that's exactly what it's there for: It's actually the answer to a simple math problem based on all the other numbers. Scanners use this to check whether they read the numbers correctly and make sure you're not accidentally paying for a flat-screen TV instead of that bottle of detergent.
Computers usually do the work, but running the math in your head isn't that complicated if you're sufficiently motivated (read: desperate for free booze). Suppose you started with this:
Bar Code Robot
Because this is a dystopian world where all beers are forced to wear labels.