Dot Physics

I got this question. How does this game work? Really, this is one of those silly things that gets forwarded a lot. It is called Regifting Robin. The basic idea is:

  • pick a 2 digit number, like 37
  • subtract both the number in the tens place and the number in the ones place from the original number
  • find the number you have left on a grid of “gifts” and robin will guess what your gift was

You may have seen stuff like this before and figured it out. It really isn’t too bad as long as you know two things (one is a trick)

  • In a two digit number, like 37, the number is really 3*10 + 7*1. I know you may be saying “duh”, but a lot of people gloss over this little detail.
  • In the gift squares, there are 100 options. Some of these are repeated.

So, let me represent this two-digit number as ab. This is:


And now to subtract a and b from ab:


There are not an infinite answers since the one’s digit cancel. There are only ten answers that you could end up with. Carefully look at the gifts you have to choose from. Here is my super-enhanced version:


Notice in this case that all the multiples of 9 have the same ‘happy face mug’ gift. Done. No magic required.

Maybe I shouldn’t give away their little trick. Well, in this case it was way too easy. Maybe this will encourage them to come up with more elaborate tricks.


  1. #1 Tom
    August 19, 2009

    I got spammed with this recently, with a whole bunch of “OMFG, this is amazing!”-type comments in the forwarded sections. I “replied all” with the answer, but it occurred to me that this is a great example of the failure of math education that so many people can be taken in by simple algebra.

    ‘You were trapped like carrots”

  2. #2 Anonymous Coward
    August 19, 2009

    Nice writeup!

    I went to the site without reading the explanation, and I’ll admit that when I first ran the thing I was blown away. I had two thoughts:

    1: Why the heck is this under the pretense of “Regifting Robin” instead of a straight-up number guesser?


    2: How the heck did that work?

    A quick look back to note that every multiple of nine was the same gift cleared up both questions simultaneously, but I was impressed with how the artfulness of the “matrix of words” camouflages the mechanism on the first pass through. I suppose that’s the basis of a good magic trick? “You chose the toaster” is much better misdirection than “you chose a multiple of nine”.

  3. #3 John
    August 21, 2009

    Thank you! You often write very interesting articles. You improved my mood.

  4. #4 Alan
    August 28, 2009

    How is this an answer? Sure there are only nine possible answers, as when you reduce numbers they ultimately are 1-9. If the answer was always “Happy Face Mug” then you would have a point. But it isn’t, each time it is different. So don’t see how this answers the mystery of how it selects the right one of 9 solutions or why the commentors above think you have answered it. Surely there is a numerical solution, but this is only part of it.

  5. #5 Alan
    August 28, 2009

    Ah, I see from another article, the layout of the grid changes so that a different gift is placed at the 9 spots, of course…..

  6. #6 TJ
    October 31, 2009

    dude, let me repreat what he said. they figured out that every number minus its own digits is a multiple of nine, so they turned it into a “guessing game”. BTW a multiple means if you multiply nine the right amount of times, youll get that number. For example-
    37-3=34 34-7=27 3*9=27 Whereas the three would be the “multiple” and the multiplications sign would be “of” and 9 is “nine” so 27 is “a multiple, 3 of, * nine, 9” if you still dont understand, go back to school, im 14 takeing highschool classes and i know this stuff.

  7. #7 MD
    December 22, 2009

    WOW THATS CRAZY!!!!!!!!!!!!!

  8. #8 tab
    January 5, 2010

    still,, math involved, It is still a matter of diff numbers that can be chosen,, so how does it know u have a mug , not a light or a broach? Still dont answer that, i understand their answers are limited but how does it know every single time EVERY single time which u choose? STILL NOT ANSWERING THAT

  9. #9 bla
    January 6, 2010

    hey still im a 6th grade kid and i dont understand! if somebody would be kind in explaining me by emailing me to ‘’ PLEASE!!!!!!!!!!!

  10. #10 John Richardson
    April 20, 2010

    I see that she asks you to do a computation that is known as casting out nines… as in 41 -1=40 -4=36 3+6=9… its always 9,the only transmitted variable is the time it takes you to do that computation, some are easy but some take a few seconds longer.I think they use an average time table that gets them very close.Actually they may have gone on line with a little gimmick test and fed the time data into a computer,after it ran for a few weeks or months they would have some very accurate data to use in this trick.She got me every time but when I didn’t do the computation and just picked any gift when the last page opened She missed it every time. The average computational time in my opinion is the key to this trick. My guess is that they are correct 85% to 95% of the time but not if you use an abacus or a calculator because they would be too fast or too slow and fall off their chart….J

  11. #11 george
    July 15, 2010

    still,, math involved, It is still a matter of diff numbers that can be chosen,, so how does it know u have a mug , not a light or a broach?
    Few basic things:
    If the specified task is applied to the numbers between 99 – 0, there are few groups of numbers, each of which gives the same result – example 92,93,94,95,96,97,98 and 99 all point to 81, as result. Same goes for any number in the table.
    So in the end, you end with only few numbers, being the possible results for all the numbers.
    Place there the special result which is “the correct guess”
    and voala. 100% correct “guess” every time.

  12. #12 Connie
    July 29, 2010

    I understand the equation part of this but what I can’t figure out is when I don’t think of a number to try and trick it, it tells me a gift that isn’t even on the grid. I always wait a few seconds to make it seem like I’m calculating. It’s like it’s telling me “aha, I know you’re not really thinking of a number”. I’ve seen a lot of these but this is the best. It’s great when you have a group of people together trying to “trick Robin”.

  13. #13 TMI
    January 30, 2011

    I found the instructions misleading. First, you are asked to pick a number. Then you are asked to do a math problem. You are not asked to remember the result of the math problem, you are asked to remember YOUR NUMBER!

    By engaging you in the calculation, you are initially misdirected and finding the correct answer at all possible results occupies you with suspension of disbelief, taking you further from the truth.

    Cute math problem but a far more elegant con! Reminds me of the questions on standardized testing that are often scored wrong because the author fails to see a glaring alternative to the intended answer.

  14. #14 jheraty
    February 8, 2011

    I saw this, & explained it to a friend this way – (Less mathematically prcise, but easier for non- mathematicians to understand…)
    “If you add the digits of any two-digit number, you get a multiple of nine. The gifts listed on each new grid are always the same for each multiple of 9, although this is not immediately obvious because of the layout of the grid.
    So, whatever number you pick, it will always come out as a multiple of 9.
    It looks clever, because the gifts change on each new grid, but in fact you could only ever pick that gift!
    It’s nothing to do with computational times!

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