#2. Quickly Calculate the Weekday for Anyone's Birthday
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.
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Disclaimer: We're assuming you're also devastatingly attractive.
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:
Along with The Wiz's 35th anniversary on October 24.
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.
#1. Digital Roots: The Ultimate Mathematical Mindfuck
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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.
And half your brain is porn, but that was true in any case.
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 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.
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.