Amazing mind-reading (?) web site

January 31, 2005 9:01 AM

Pretty slick mind-reading trick... Perhaps wizened MetaFilter readers will see through its inner workings, but to me, this site just looked like magic. The page vanishes after a minute of disuse, so you may need to link more than once. (My first post on MF.)
posted by humannature (36 comments total)

Pretty slick if you haven't taken an algebra class.

11: 1 + 1 = 11 - 2 = 9
12: 1 + 2 = 12 - 3 = 9
13: 1 + 3 = 13 - 4 = 9

notice a pattern?
posted by milnak at 9:05 AM on January 31, 2005

Explained this one to a friend long ago, at least I think I explained it correctly:

So here's the solution.
No matter which two digit number you start out with the only possible solutions are:
0, 9, 13, 18, 27, 31, 35, 36, 45, 54, 63, 72, 81, 88, 99

If you look, the symbol next to each of these numbers is the same each time. So you're going to see that symbol as the solution. Each time the game cycles, the symbols next to the numbers change, so the symbol next to 0, 9, 13, 18, 27, 31, 35, 36, 45, 54, 63, 72, 81, 88, 99 will be different. Yet the symbol next to those numbers is always the symbol you will get.

Now, I'm pretty much innumerate, so I have to rely on smarter mefites to validate or call foul on my explanation.
posted by Pliskie at 9:05 AM on January 31, 2005

An ROT-13 spoiler: Yrg gur gjb qvtvg ahzore or "no", gung vf gra gvzrf n cyhf o. Gura gur ahzore gung erfhygf sebz gur pbzchgngvba nfxrq sbe vf (gra gvzrf n cyhf o) - (n + o) = avar gvzrf n. Nyy bs gur ahzoref va gur tevq gung ner zhygvcyrf bs avar unir gur fnzr flzoby: ibvyn!
posted by muhonnin at 9:09 AM on January 31, 2005

Pliskie's right.
The attached symbols change each time, so it doesn't feel like you're doing the same thing over and over. Since Pliskie's solution table is the only set of solutions you can possibly come up with, you will invariably hit the symbol chosen to associate with 0, 9, 13, 18, 27, 31, 35, 36, 45, 54, 63, 72, 81, 88, 99. Watch, every time you do it, the symbols on those numbers will match each other.
posted by dougunderscorenelso at 9:10 AM on January 31, 2005

Suppose your number n = 10x + y.
The sum of the digits will then be x + y.
Subtracting them from your number will be 10x+y -(x+y) = 9x.
All the symbols that are at multiples of nine are the same.
posted by smackfu at 9:10 AM on January 31, 2005

I don't think the 13, 31,35, etc. can come up - the solution always returns a multiple of 9, which, as you correctly say, all have the same symbol by them.
posted by benzo8 at 9:11 AM on January 31, 2005

(On preview: Which makes sense to me, but how do you get 13 or 31 as a possible solution?)
posted by smackfu at 9:11 AM on January 31, 2005

I'm guessing Pliksi reverse engineered it - looking for all the numbers with the same symbol, and I'm guessing too that the game designers threw some red-herrings in there...
posted by benzo8 at 9:16 AM on January 31, 2005

Excactly, benzo8, my solution was not math-based. Like I said, I'm innumerate. I noticed that all those numbers are grouped to the same symbol each time, and then I did a quick test to see if the bulk of them were valid solutions for the equation. Some were, and I generalized the rest together. The fact that all the valid solutions for the equation are multiples of 9, well that's just too mathematical an approach to dawn on me first.
posted by Pliskie at 9:17 AM on January 31, 2005

Just to clarify the warning in the post, when the page "disappears", it closes my entire browser (Firefox 1.0/WinXP) including all other open tabs. Very annoying to say the least.
posted by grapefruit at 9:18 AM on January 31, 2005

Amazing how quickly Pliskie seized upon the right solution... and how quickly milnak seized on a completely WRONG one (his "pattern" ends at 19). Pliskie's head recognizes patterns, and he's self-aware enough to know his strengths and weaknesses. Very impressive.
posted by humannature at 9:34 AM on January 31, 2005

I could swear we've looked at this before on MeFi, but I'm not calling double-post because I can't find it.
posted by beagle at 9:44 AM on January 31, 2005

This has been posted but the old one was a different link.
(the one I remember was on a white background) No harm no foul though I doubt you'd find it easily using the search.
posted by bitdamaged at 9:48 AM on January 31, 2005

Uhm, humannature, you have it the wrong way round. Pliskie's solution is the wrong one (his pattern contains crud like 13 and 31) and milnak's is completely correct.

What amazes me is that anyone with more than a grade school education would take more than 5 seconds to figure out this 'trick'. It's really, really basic math after all.
posted by Deepspace at 9:48 AM on January 31, 2005

What amazes me is that anyone with more than a grade school education would take more than 5 seconds to figure out this 'trick'. It's really, really basic math after all.

Well, condescension aside, it took me more than 5 seconds, but it wasn't because of the math. It took me a few tries to realize they were changing the symbols.
posted by pardonyou? at 9:53 AM on January 31, 2005

Well, if you want to be exact about it, milnak's solution didn't produce the series far enough to show that the solution was multiples of 9, rather than just "they're all 9", and it didn't go on to explain that those numbers all had the same symbol beside them. So it wasn't completely correct, by dint of not being complete.

So, credit where it's due, the first correct post was munhonnin's ROT13 encrypted solution.
posted by benzo8 at 9:53 AM on January 31, 2005

Nice post.
posted by OmieWise at 9:54 AM on January 31, 2005

I knew there was a trick to the numbers being multiples of the same thing but I got *completely* suckered by the far more obvious trick of changing the ringer symbol each game.

Does this mean I have to bow before Deepspace's annoying superiority?
posted by CunningLinguist at 9:55 AM on January 31, 2005

Deepspace, milnak was wrong in that he did NOT offer an explanation, but instead merely implied that all solutions would add up to 9 (as noted by benzo8). This is incorrect, and also far from a solution. A COMPLETE solution has to include the observation that the symbols change so that the trick works. Pilskie, and NOT milnak, observed this and pointed it out. It is entirely possible that milnak KNEW the solution. But only Pilskie actually articulated and PROVIDED one. Which probably reveals a bit about each of them as human beings - my real interest here anyway.

As far as this post appearing before, I was surprised that I could not find it, but I did do my due-diligence search, using a number of different keywords (crystal ball, mind-reading, magic, number trick, etc.). This yielded no results, so I posted.
posted by humannature at 10:09 AM on January 31, 2005

It's surprising how often this one comes up. I came close to posting this myself (different version, I think) a month or so back when this hit my office as a viral e-mail. Luckily, the URL of the version I searched on did come up in the search function.

I did have great fun impressing my co-workers with my amazing (MetaFilter-based) math skills.
posted by mmahaffie at 10:10 AM on January 31, 2005

Plus, this version has that great transporter sound effect from old school Star Trek.
posted by steef at 10:14 AM on January 31, 2005

The trick is an old one, and easy if you're in the practice of thinking about math symbolically, but it does my heart good to see people discussing and playing with this who wouldn't otherwise give two shakes about algebra. So, not a bad post at all.
posted by Wolfdog at 10:15 AM on January 31, 2005

humannature, just FYI, the easiest way to look completely barking mad is to start capitalizing lots of words. You just reminded me of the loonmail we get at the office. Fun post though.
posted by CunningLinguist at 10:18 AM on January 31, 2005

CunningLinguist, yeah you're right. Sorry about that.
posted by humannature at 10:20 AM on January 31, 2005

Screw you nerds, I liked it.
posted by Uther Bentrazor at 10:44 AM on January 31, 2005

steef: right on, but to be more specific, it is the planet-side sound of the transporter. It's a different sound when you're actually in the transporter room.
posted by mystyk at 10:57 AM on January 31, 2005

Just wait until someone tells them about the n-dimensional hypervolume, that'll really melt their brains.
posted by StrangerInAStrainedLand at 11:36 AM on January 31, 2005

What amazes me is that anyone with more than a grade school education would take more than 5 seconds to figure out this 'trick'. It's really, really basic math after all.

Porcupine!
posted by gagglezoomer at 12:04 PM on January 31, 2005

I like that 90 and 99 aren't given the same symbol as the other (achievable) multiples of nine.
posted by Aknaton at 12:41 PM on January 31, 2005

But only Pilskie actually articulated and PROVIDED one.

but however his solution, while close, was wrong.
posted by bitdamaged at 1:25 PM on January 31, 2005

I think last time around it was with David Copperfield. The FPP referred to a card trick, and the symbol trick I think was on a related page. But the old link doesn't work anymore.
posted by Doohickie at 1:32 PM on January 31, 2005

Muhonnen: Nice use of rot13.
posted by Calast at 2:39 PM on January 31, 2005

More here, here, here, here, and here
posted by growabrain at 8:31 PM on January 31, 2005

Ha! It's not so smart. It was completely wrong about which one I chose.

. o O (Wait-- What's all this talk about it being a sure thing?)
posted by Laen at 8:41 PM on January 31, 2005

You guys are amused by the various attempts to provide an explanation (false-starts and otherwise).

I'm more amused by how so many of my "spiritual but not religious" friends swear that it's not just some math trick, and that there is real psychic power behind it. I'm serious. Come to think of it, this isn't amusing, it's saddening.
posted by randomstriker at 10:16 PM on January 31, 2005

There used to be a cunning version of this in an Excel spreadsheet. It gave the same instructions to get your final number but then directed you to scroll down the list of the symbols on the right hand side and then click the button on the left hand side which would reveal the image you were thinking of. The clever bit was that the list only showed nine symbols at a time, and a vbscript recorded what nine you were looking at and then it knew what symbol you were looking at when you clicked the button.
posted by dodgygeezer at 11:39 AM on February 1, 2005