# 21-card trick – Numberphile

Numberphile

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A simple trick with some neat math behind it. More on Numberphile cards at:

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More cards video:

This video features Anastasia Chavez:

We did a similar video to this about six years ago with Matt Parker (and 27 cards):

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28.08.2023

A fun variation on this trick:

Count down 7 cards in a pile, then the next 7 cards you're going to alternate between 1/3 down and 1/3 up from the previous pile, so they lock in as if you were doing a riffle shuffle (1/3 overlap in the middle), the next 7 are going on top in a normal pile. You now have a pile with 4 cards sticking out the bottom and 3 sticking out the top. Grab the pile and squeeze the sides so that the pile is slightly bent. Now press the side with the 4 cards down on the table so the 3 cards on the other side rise, turn the pile upside down and press the side with 3 cards down on the table, turn once more (there should be 2 cards sticking out the bottom) and press down while exclaiming: "Was this your card?"

The sound of the cards being shuffled and dealt is perfect ASMR material

can you hold your camera steady??? It ruins everything

Beginning of every card trick:

Magician:"Just a bunch of cards" or "Please confirm this is an ordinary deck of cards"Audience:WHOOOAAAAU could force that card would be so much easier

This has been my go-to card trick since I was a kid. I've never thought about the math behind it but it makes sense.

Wow this is the worst acting i saw

I picked the same card. Wack

the magic is gone… 🙁

the end is a trick for a 2 year old

This is like encryption of data.

Completely amazing

This skipped over selecting the piles of three and how to guarantee the right one is chosen. 🤨

WHAT IF THE GUY CHOOSES THE PILE IN WHICH THE CARD IS. I THINK THE END IS ALL LUCK

so this is what professors do during office hours

i didn't get the last step. 😑

This is the first card trick that i learnt , and this inspired me to be a magician

hi very well performed iam ace card magician from uk

I do this trick but with a twist – I make the card rise out of the deck.

Matt parker already did this one a few years ago.

This is the trick my dad taught me when I was a kid. It's not a trick actually it's Math 😁

What if the spectator picks the pile (to be removed) with their card number?

Looking for a card trick I learned in jail. Cut deck in half pick card stick it in middle. Flip over a card and if it’s an 8 put down 2 cards again if it’s a 3 put down 7 and again if it’s a 5 put down five. As up cards and that’s card that was picked in beginning. Idk how it works. I did it for years but can’t do it any more and don’t exactly remember how it was done but it blew my mind even when I did trick my self

Very poor explanation for this.

Give written formula in form of mathematical equations.

idk how nobody asked this, but at the very end when you had them pick their card out of the last three, how did you make sure it was their card? i know it’s a one in three chance but still

Take a sec to appreciate that shuffle sound

This is the first card trick I ever learned, back in tenth grade.

its working with 33 cards and 17s selected card

Given that each location – left/centre/right – defines a ternary number 0/1/2 – and that there are three deals each yielding a ternary number – presumably the trick still works if, instead of dealing 21 cards, 3^3=27 are dealt.

What if he chooses the group that has that card?

Sweet!

My mom's prison boyfriend showed me this when I was 10 years old I thought it was so cool.

First card trick I learned, but never really felt it was a "proper" trick since it was just maths.

🙂

Please translate in Hindi 👍

Please in Hindi 👍

2:55, what is this 4 of hearts of diamonds at the top/right?!

The first trick I ever learned. Learned it from a Mexican in Cabo when I was like 11. it blows people's minds if you do it right. I've always wondered what the math is behind it, thanks for making this video!

My dad used to do this trick for me 30+ years ago when I was a kid

🌹🌹🌹🌹🌹🌹🌹🌹

but why is it always the 11th card

Nice. Nice trick.

Can u do the 51 card trick

I've know this trick about 40 years, you can change the ending to suit the effect if you like;

11

Nice smile!

It worked 🎉

51 cards is amazing than 21😊

I learned it a much different way

Completely overcomplicating a very very easy trick to do, so very hard to understand