An Odd Card Trick


116 points | by allthings 9 days ago


  • wwwwewwww 8 days ago
    This reminds me of a self-working trick I saw online in the 90s.

    The website shows you a hand of 5 cards, and asks you to pick one card and stare at it intensely. It claims it will read your mind and guess the card. When ready, you will click a button, and 5 new cards are shown - 4 random cards plus the card you thought of.


    The trick is that the same cards are shown as before, but in different order. Most people only focused on the one card and don't notice that the rest of the cards are the same. It sounds very simple, but it worked on me and all my family members, at least the first time.

    • tinus_hn 8 days ago
      I have seen the same concept where they show 5 cards, ask you to choose one and then click the button.

      Then it shows the other 4 cards, with your card removed.

      In reality of course it’s a similar set that contains none of the same cards.

      • BigJono 8 days ago
        Do you remember what site it was on? It sounds super familiar.
      • FatalLogic 8 days ago
        It seems like the basic descriptions of these two tricks, the cards and the liquids, are terribly vague and imprecise in this article, so it's difficult to discuss it. In particular, it's unclear what information is shared, and what information is hidden.

        The logical and mathematical analysis later in the article looks more detailed, but unfortunately the initial, fundamental, description of the process lacks clarity and forces us to guess the details

        • jandrese 7 days ago
          I have to admit I still don't understand what is going on with the water and wine part.

          The card trick is pretty straightforward though. When you mix up the cards you end up with something like 7 face up and 3 face down in one deck and 3 down 7 up in the other. The trick is just that the magician flips his cards over, inverting the cards and matching the other deck. Presumably he does this while engaging in some misdirection.

        • wellpast 9 days ago
          Just performed this trick for my partner and she was very underwhelmed. There’s probably a good delivery to figure out.
          • fenomas 9 days ago
            TFA explains the trick somewhat oddly, to make the math angle more obvious. Here's a (random) video of someone doing it a more usual presentation of the effect:


            Not the world's greatest magic trick or anything, but I don't think it's as transparently obvious to figure out as several commenters are suggesting.

            • tibbetts 9 days ago
              Linking to a 9 year old YouTube video with 36 views and 12 subscribers should have an amusing impact on their metrics.
              • loonster 9 days ago
                I was going to skip the video, but after knowing the metrics I needed to add to the level of bewilderment.

                The video has been up for ~80k hours. It just got 80% of its views in the last hour.

              • cyounkins 9 days ago
                The article describes a fixed number of cards to the performer and spectator, but the video has him split the deck behind his back. Is he counting by touch to have the correct number of cards?
                • fenomas 8 days ago
                  Yes. That's what all the "but it's obvious!" commenters are missing - normally the spectator wouldn't have all the information that TFA lays out (that exactly half the deck starts out reversed, exactly how the cards are split up later, etc).
              • rnoorda 9 days ago
                Having dabbled in magic some, I can confirm that the delivery is far more important than I originally thought. Several times I have had my mind absolutely blown by a card trick, then found out it's a trick I have done myself! But they had put in so much more effort on honing the delivery, presentation, and patter to the point it became a different trick.
                • nickpeterson 8 days ago
                  This is so important in most fields. Marketing works.
                • bena 9 days ago
                  Work it with a mentalist angle.

                  Separate the cards into a face up and face down, shuffle, deal out two piles while explaining that you can read their mind. That you can pull numbers right out of their head.

                  Have them pick a pile. Slide it over to them and slide one under the table while telling them so you can't see what you're doing. But really it's because they can't see what you're doing.

                  Tell them to go through their pile and count the number of face up and face down cards. But don't tell you. Emphasize that they shouldn't tell you the numbers. Once they have the counts, stare at them, make some jokes about what they're thinking. Then shuffle the cards around under the table a bit. Make it look like you're thinking about what you're doing, mutter, "yes, yes, no, up, down" etc.

                  Then after a few seconds, say you got it and do the reveal.

                  Bonus, if they flipped their deck before counting and therefore the counts are backwards, you can go "ta-da" and flip your deck in front of their face, working the misfire as a false reveal.

                  • mhitza 9 days ago
                    For an impressive self working trick I can recommend this video .
                    • jedimastert 9 days ago
                      The magician term is "patter", and it's just as important as the technique. Often some of the best tricks are just very good stories, and the technique is the storytelling device.
                      • numlocked 9 days ago
                        Yes - it boils down to “there are 10 cards. Pick some number of cards, give me the rest, and I’ll tell you how many you picked”. The addition of “more cards” very thinly disguises the basic fact that you are subtracting a number from 10. It’s not obvious to me how this could be done in a way that seems impressive.
                        • FartyMcFarter 9 days ago
                          > Yes - it boils down to “there are 10 cards. Pick some number of cards, give me the rest, and I’ll tell you how many you picked”.

                          It boils down to that when you know the trick, but it isn't trivial to conclude that from just seeing the trick being performed.

                          • Marazan 8 days ago
                            Do the whole trick blindfold. And learn some patter.
                          • ummonk 9 days ago
                            Probably have to add some elaborate narration and then some sort of routine in the process of flipping your own cards so they forget how things started out.

                            Also, given that you're on HN, you've probably chosen a partner who is at least somewhat intellectually stimulating and thus significantly more likely to think this through than the average person.

                          • alasdair_ 9 days ago
                            The wine and water explanation doesn’t make sense to me. Wine and water have different molar masses and combining a liter of water and a liter of wine doesn’t give exactly two liters of the new substance, in the same way as a liter of ping pong balls and a liter of sand doesn’t produce two liters of ping pong sand when mixed together - some of the molecules are bigger than some of the others.

                            Once you pour a liter of the new, mixed, wine-water substance back into the first container, you’ll likely have less than a liter in the second container, so the wine bottle will contain more than the container originally containing water. Right?

                            • dreamcompiler 8 days ago
                              If you mix 1 liter of pure alcohol with 1 liter of pure water, you'll end up with less than 2 liters of liquid. But wine is a little bit of alcohol already mixed with a lot of water, so the "interstitial molecular crowding" will have already happened. So 1L of wine + 1L of water produces 2L total.
                              • gus_massa 9 days ago
                                The article is about mathematical water and mathematical wine. They have additive volumes.

                                Real water and real wine are more complicates. More generally, after mixing some water and some ethanol the volume of the mix is smaller than the sum of the volumes. And this is not only a theoretical result, it's possible to measure the difference experimentally

                                • nkrisc 8 days ago
                                  Fine, replace the water and wine with two different liquids that have exactly the same molar mass.

                                  You’re missing the point of the example by thinking of it as real. There’s no actual water or wine here.

                                  • graup 9 days ago
                                    I think you made the same mistake that I made when I first read it. The question isn't "what contains more wine?" but "compare the amount of water in A to the amount of wine in B."

                                    Step 1: A with 10 L wine; B with 10 L water

                                    Step 2: A with 9 L wine; B with 11 L liquid (10 L water and 1 L wine => 10/11 water, 1/11 wine)

                                    Step 3: A with 10/11 * 1/11 = 10/11^2 L water; B with 1/11 * 10/11 = 10/11^2 L wine.

                                    10 / 11*2 water in A ~ 10 / 11^2 wine in B.

                                    • daveFNbuck 8 days ago
                                      You're misunderstanding the objection. They're saying that if you move 1L of wine to the water side, the side you poured into will have fewer than 11L and when you pour 1L back the two sides won't have equal volume.
                                    • ZeroGravitas 8 days ago
                                      Excluding the physical elements, it still seems a poor, possibly actively misleading (or should I say misdirecting), analogy.

                                      The better way to understand it is to use only two cards, one face up one face down, if you give one to each person randomly, how can you magically match the other person? By flipping the card, because for every down you have, the other person has one up.

                                      • shawabawa3 8 days ago
                                        You are being pedantic

                                        Most people won't consider the sub ml change in volume

                                        • GoldenGateBRDG 8 days ago
                                        • d--b 9 days ago
                                          No, it’s not like ping pong balls and sand. A liter of water and a liter of wine definitely make 2 liters of liquid…
                                      • xattt 9 days ago
                                        The site has more tricks up its sleeve. Right-clicking on mathematical equations brings up a MathJax context menu!
                                      • rawoke083600 8 days ago
                                        Lol cool. I wonder if some of these types of card-tricks methods - based on "math and shuffle-rounds etc" to get a desired "result/order", can be used in a type of DB-Indexing ?

                                        There must a 100+ of these types of tricks or algo's

                                        • dghughes 8 days ago
                                          I'm always amazed at that a 52 card deck can be arranged in 8x10^67 different ways.
                                        • ummonk 9 days ago
                                 since it has the Hacker News hug of death.

                                          This is such a ridiculously obvious trick I'm surprised any audience is impressed by it. But that happens for me with a lot of magic tricks... I guess a lot of people just choose to shut off basic thinking (and in this case don't have an intuitive understanding of conserved quantities).

                                          • asxd 8 days ago
                                            This is often called "suspension of disbelief" and is an integral part of becoming immersed in some forms of art. When you watch a movie, for instance, you know that none of it every really happened. You probably can guess the tricks they used to create the visuals. But constantly reminding yourself of that fact tends to ruin the illusion and the fun.
                                            • jonnybgood 9 days ago
                                              > I guess a lot of people just choose to shut off basic thinking (and in this case don't have an intuitive understanding of conserved quantities).

                                              Or maybe these people just want to be entertained and not waste mental energy on something like a magic trick.

                                              • ummonk 9 days ago
                                                > Or maybe these people just want to be entertained and not waste mental energy on something like a magic trick.

                                                Yes, presumably that's why they choose to shut off basic thinking