5 Amazing Wizard Tricks You Can Do With Basic Math

The Magic:

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.

The Math:

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:

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.

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.

The Magic:

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.

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!

The Math:

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:

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.

The Magic:

Somehow, you learn the birth date of someone you want to impress. Maybe you're carding her at a 7-Eleven, or getting her license and registration for speeding. Either way, you read the date and absent-mindedly add, "Ah, a Saturday." She looks it up on her phone and, astonished, asks how you calculated that so fast. You turn pale. "D-Did I say that out loud? Please, you must tell no one. I can't go back to the laboratory."

Soon, you're having freaky sex. It cannot fail.

The Math:

You don't have to be very smart to calculate the day of the week for any date -- not when eminent mathematician John Conway already devised a clever shortcut for that, which he (somewhat overdramatically) named the doomsday rule. Crack open a 2014 calendar and you'll see that 4/4, 6/6, 8/8, 10/10, and 12/12 all land on the same weekday: Friday. The same goes for other easy-to-remember dates like 5/9 and 9/5, 7/11 and 11/7, the last day of February, Pi Day, July 4, Halloween, and Michael Jackson's birthday (August 29, as you know). Again, all Fridays.

In 2013, those were all Thursdays:

In 2012, they were all Wednesdays. Starting to notice a pattern? Since 2012 was a leap year, in 2011 the "doomsday" jumped to a Monday:

And so on. So, let's say you're trying to find out the day of the week for July 9, 1987. First you have to figure out the doomsday for that year, by using an important world event as a point of reference, for example. As you probably learned in history class, Captain EO came out on Friday September 12, 1986, so from there it's easy to calculate that 9/5 was a Friday, too. If 1986's doomsday was a Friday, then 1987's was a Saturday, which makes July 4 of that year a Saturday as well. Therefore, a quick finger count tells us July 9, 1987 was ... a Thursday.

The trick is so easy that Conway was able to mentally calculate any date in less than two seconds. We're assuming he died from a vagina overdose.

The Magic:

After doing all the tricks above, your friends and new sex partners demand that you tell them the truth about how you're so damn good at math. So you tell them: One time, lightning struck your computer, and your brain temporarily fused with the Internet. Your mind gained supercharged computation powers, but because it's the Internet, you only use them to tell other people they're wrong.

To demonstrate this, you tell them to grab a calculator, multiply random four-digit numbers, and write down the results, but intentionally write some of them wrong. Common mortals wouldn't be able to tell the difference, but you're not a common mortal, so you briefly glance over each problem and tell them which ones were changed. They never doubt your powers again and become your butlers.

The Math:

The secret is so ancient that it was already old by the 10th century, and so useful for double-checking your homework that you might have learned it in school and just forgot. Let's take the following problem:

Is that result right or wrong? No fucking clue. Unless, that is, we were to make all those scary numbers more manageable by simply adding up their digits:

Those are the numbers' digital roots -- sort of like the spokesperson of each number that can represent it when it can't be bothered to leave the house. So, instead of multiplying the big four-digit figures, we can multiply the roots instead. If the roots of the question and the answer match, congratulations, the answer is right! If not, then someone's lying or wrong.

This method is called casting out nines, and there's an even bigger mindfuck you can pull off with it: Tell your spectators to take any long number (let's go with 6,969), rearrange it to make another number, and then subtract the smaller number from the larger one (9,966 - 6,969 = 2,997) -- the digital root will always be 9, for some reason. Now instruct them to multiply the result by another figure, as big as they want (say, 8,008,135). Finally, have them choose one digit as their secret number, and give you the others.

They'll be like "I just spent half an hour mixing, subtracting, and multiplying random shit. No way you can guess that number." Unless you add up all the digits and subtract the result from 9 (because that's still the root), like this:

Boom, secret number. You can draw some alchemical symbols or something to spruce up the presentation, but we're guessing it won't be necessary. They'll be scared enough of you as it is.

Mr. Yee is COUNTING on you to check out his daily comedy fortune cookie and assortment of amusing merchandise. Can you DIGIT?

For more ways to make yourself look smart, check out 36 Awesome Low-Tech Tricks Used in Movies.

Related Reading: Alas, some math IS useless in the real world- like long division. But your arithmetic skills can come in handy in places you'd never expect, they could even help you write a pop song. And if you'd like math's help cooking the perfect pancakes, you need only read on.