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This content is taken from the Davidson Institute of Science Education at the Weizmann Institute of Science's online course, An Introduction to Recreational Math: Fun, Games, and Puzzles. Join the course to learn more.

Skip to 0 minutes and 0 seconds Are you ready for our first magic trick? Cards are great for magic tricks. Here’s a card trick attributed to Martin Gardner. And to help me with this magic trick, Is Orel, Hello Orel!

Skip to 0 minutes and 13 seconds Orel: Hi!

Skip to 0 minutes and 14 seconds Yossi: Orel is very shy and, so we’re going to do the shy card trick! and so I’ve taken here seven cards from a deck of cards - check them - they’re regular playing cards,

Skip to 0 minutes and 27 seconds Yossi: Yes?

Skip to 0 minutes and 27 seconds Orel: Yes! No special whatever?

Skip to 0 minutes and 28 seconds Orel: No!

Skip to 0 minutes and 28 seconds Yossi: Good! So, now, what I’d like you to do is I’d like you to choose one of these cards. Choose whichever card you like. Any card. Show it to the camera but not to me…

Skip to 0 minutes and 40 seconds Orel: OK!

Skip to 0 minutes and 40 seconds Yossi: OK?

Skip to 0 minutes and 40 seconds Orel: Huh huh! and, OK, so give it me back… …and by the way these cards they’re regular playing cards there’s no markings on them whatever, Now, for this card remember it’s the shy playing card trick… I’d like you to choose a number, any number.

Skip to 0 minutes and 59 seconds Orel: Five

Skip to 1 minute and 0 seconds Yossi: Five!

Skip to 1 minute and 0 seconds Orel: Yes!

Skip to 1 minute and 0 seconds Yossi: Are you sure?

Skip to 1 minute and 0 seconds Orel: Yes!

Skip to 1 minute and 2 seconds Yossi: You want to change?

Skip to 1 minute and 2 seconds Orel: No!

Skip to 1 minute and 2 seconds Yossi: No! Five! We’ll go with five! Ok! So what I’m going to do now… is I’m going to count every fifth card and expose its face. and you’ll see that your card is so shy it will never expose its face.

Skip to 1 minute and 21 seconds Yossi: OK?

Skip to 1 minute and 21 seconds Orel: OK!

Skip to 1 minute and 21 seconds Yossi: So let’s go then! 1,2,3,4,5… and this is not your card right?

Skip to 1 minute and 33 seconds Orel: No!

Skip to 1 minute and 33 seconds Yossi: No! It can’t be your card because your card is… the shy card, right? so we’re going to keep this card face up to remember that its already shown its face OK! and let’s continue… 1,2,3,4,5… and this is not your card either, right? because your card is the shy card. and we’re going to keep this card face up. 1,2,3,4… and is this your card? …5, is it?

Skip to 2 minutes and 7 seconds Orel: No!

Skip to 2 minutes and 7 seconds Yossi: No! It can’t be because your card is the shy card it won’t show its face. OK! Let’s carry on! 1,2,3,4,5 - is this your card?

Skip to 2 minutes and 22 seconds Orel: No!

Skip to 2 minutes and 22 seconds Yossi: No? It’s not the 4 of hearts? No! because your card is the shy card. 1,2,3,4 - is it this card?

Skip to 2 minutes and 33 seconds Orel: No!

Skip to 2 minutes and 33 seconds Yossi: No! It’s not your card… No! See it’s… in fact…yes…well there are two cards left So, do you think it’s going to be one of them? Let’s see… 1,2,3,4… is it this card? The eight of diamonds?

Skip to 2 minutes and 47 seconds Orel: No!

Skip to 2 minutes and 47 seconds Yossi: No! Now isn’t that funny, you see… …all of the cards have exposed their faces… except for the shy card, the Jack of Spades!

Skip to 3 minutes and 5 seconds Orel: Oh my God! How do you do that?! You’d like to know how I do that?

Skip to 3 minutes and 9 seconds Orel: Yes!

Skip to 3 minutes and 9 seconds Yossi: Ah ha! It’s a magic trick - I’m not going to tell you! But, it’s up to you guys… to try and find out how I do these magic tricks! They’re all based on math! Want to try?

A Mathematical Card Magic Trick

Now for a surprise! At the end of each week I’m going to perform a magic trick that is based on math. It’s your job to try and find out the secret behind the trick. Post your ideas below! The correct answer will be disclosed in the weekly email…

If you’ve come so far, you probably enjoy the course. Bear with us for next week, where we’ll delve into some specific topics in recreational math. Now, I’ll dive into the comment to see whether anyone found the right explanation for the magic trick…

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This video is from the free online course:

An Introduction to Recreational Math: Fun, Games, and Puzzles

Davidson Institute of Science Education at the Weizmann Institute of Science