Math has a lot in common with sorcery: Using vast sources of ancient knowledge passed from generation to generation, it helps us compel the forces of nature to do our bidding just by scrawling out arcane diagrams and incomprehensible symbols on a whiteboard. Also, both will guarantee that you won't get a second date if you spend the first one talking about them.
While only the highest Numerati may understand the spellcraft that makes our modern wonders possible, even a Level 1 Math Wizard wielding basic fourth grade arithmetic can pull off seemingly impossible feats. Here are a few easy but impressive tricks that you can use to intimidate your friends and amaze your enemies.
#5. Perfectly Sort Coins Without Looking at Them
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Let's start with a trick a moron could do. You can do it with coins, or playing cards, or anything with two marked sides. Do it with dollar bills and impress a stripper, whatever.
Let's say we're using a handful of coins. First you close your eyes (or get blindfolded) and tell your spectators to shake up the coins and throw them on the table. All you need is one piece of information: how many of the coins are facing heads-up.
For your spectators' sake, start with a small pile.
Then, without ever peeking at the coins, you sort them into two piles, blindly flipping and shuffling them as if your hands were being guided by the spirit world. You magically wind up with the exact same number of heads in each pile. Every time.
Let's say your spectators tell you that there are six heads-up coins in the pile. All you need to do is grab that number of coins and flip them. Just any six random coins. Take the ones you flipped and move them to their own pile, which we'll call Pile #1. The remaining coins are Pile #2. Both piles will contain the same number of heads.
Why does this work? Because math, that's why. Say you have 12 coins total. Your friend tells you six are heads, so you flip and separate six random ones -- if two are heads and four are tails, you're still left with four heads in each group. Here, we made a chart:
Pro Tip: Don't live in one of those weird countries with coins that have heads on both sides.
Go on, take a hammer to your swear jar and try this. If you're accused of feeling the coins as you go, just repeat it with cards or driver's licenses. And if they still think your super-sensitive fingers can distinguish the images you're touching ... well, just let them think that.
#4. Correctly Guess the Final Number for Any Bar Code or Credit Card
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.
First, add up all the digits that aren't obscured by an MS Paint-made squiggly line. Then, starting from the right, add the first, third, fifth, and so on to your total. Then repeat that step. By now you've added every odd digit three times and every even digit once (you can just skip to that if it sounds simpler for you). Finally, subtract the last digit of your total from 10, like the handy chart below says -- and voila, that's your check digit. Cue incredulous, slightly terrified looks.
Bar Code Robot
Drink math responsibly.
And if you're still not done messing with everyone's minds, a similar but slightly more complicated calculation can be used to figure out the last number of any credit card. It's trickier to work out mentally if you're already half past drunk, but do it right and your friends will start wearing tin foil hats around you to protect them from your obvious psychic powers.
#3. Fake a Mystical Literary Connection With Anyone
At your local bookstore, you spot an attractive somebody carrying the exact same book as you, because it's the only book they sell (it's a bad bookstore). He or she seems a little skeptical when you say this means you must be soul mates, so you offer to prove it with a magical gesture worthy of a John Cusack rom-com.
Pick a starting page, and each of you secretly choose any word from the first line. Silently spell it out and move one word forward for each letter. So if you start with the word "throbbing," move nine words forward; if you land on "moaned," move six words. Do that until you reach the end of the page.
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"Wait, does 'foot-fisting' count as four or 11?"
At the end, announce the word you landed on: "explodegasm." Amazingly, even though you didn't know which word the other person started from, your journeys ended on a mutual explodegasm. It must be destiny!
Or, you know, just simple arithmetic. The secret is based on the Kruskal count, which isn't the name of a magic-wielding vampire, but of a mathematical parlor trick discovered by physicist Martin Kruskal. Scientists have been using this for decades to hit up chicks, while others use it for the slightly different purpose of proving the existence of God. Essentially, no matter which word you started from, if the text on the page is long enough (don't do this with a Dr. Seuss book), you'll both hit the same word at some point, and from then on the paths will be the same.
Magicians use this all the time. Here's the Kruskal method illustrated with cards -- yellow and blue start separately but inevitably meet up on the four of clubs:
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"So maybe you and I should inevitably meet, at four, at the club, eh?"
So really, your amazing connection was based less on some mystical love voodoo and more on cold, mathematical certainty. It's just like using OKCupid.