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A brain teaser is a puzzle considered to be difficult, often a lateral thinking puzzle or a logic puzzle. Not all puzzles are brain teasers; crossword puzzles and jigsaw puzzles wouldn't be considered brain teasers, though they are often difficult, while riddles would be. Sometimes, a 'tavern puzzle' is a brain teaser, sometimes it isn't.
Some example brain teasers

My grandson is about as many days as my son is weeks, and my grandson is as many months as I am in years. My grandson, my son and I together are 100 years. Can you tell me my age in years?

Solution: This puzzle attempts to generate the ages of a father, son, and grandson. From the data, you know that the grandson is a given number of days old -- we'll call it x. Then you learn that the son happens to be x weeks old. Next, you learn that the father is as many years old as the grandson is months old. This figure would be (x/(365.25/12)). Finally, you learn that their total age in years is 100. This information can be used to produce this equation (written in father, son, grandson order in years), which then can be solved for the answer below:

    x              x          x
----------- +  ---------- + ------ = 100
(365.25/12)    (365.25/7)   365.25
x is 1826.25, making the grandson five years old, the son thirty-five, and the father sixty.

What is order of the following numbers: 8 5 4 9 1 7 6 10 3 2 ?

Solution: This puzzle is easy if you write the numbers out as words:
eight five four nine one seven six ten three two

As you might now see, it's alphabetical.

0137's puzzle requires you know what the set of integers is (it's {..., -2, -1, 0, 1, 2...}). If you don't add 35 to the sum, then the sum will be -35 because all the others will cancel out.

1, 11, 21, 1211, 1231, 131221, 132231...
What is next number in the series?
Better yet, what is the last number of the series?

Solution: This puzzle is a problem of examining the series. Each number lists the digits of the number that came before. You start with 1 (one). You then have 11 (one one). You then have 21 (two ones). Therefore, the number after 132231 is 232221. The last number of the series? I wonder what the author is thinking with respect to this. Either he means an endpoint/infinitely repeating number, or the final digit. I cannot produce the former after a small amount of thought, but it should be obvious that the latter is obviously 1. Update: Sverre points out there is a last number. Let's see what it is by continuing to expand the series:

1, 11, 21, 1211, 1231, 131221, 132231, 232221, 134211, 14131231, 14231241, 24132231, 14233221, 14233221, 14233221, ...
As you can see, the number 14233221 is the final number that will appear in the series, as it is self-describing.

TaintedTex's puzzle has been done to death on the other node. Basically, you create two equations -- there's more than one solution in this case.

h = # of hens
r = # of roosters
c = # of chicks

1*h+5*r+.5*c = 100
h+r+c = 100

Other Brain Teaser Nodes

Here are some brain teasers that have been noded. If you come across any I haven't found yet, /msg me. If enough brain teasers come up, they'll have to be subclassified. I personally would say that Old Chestnut is a subclass of this -- I know I've never seen Planet X Fingers before.

I decided that having the solutions below the first list might cause some problems in location, so I have solution nodes after a double dash.

Brain Teaser Commentary

After I wrote up some solutions to brain teasers on here, I was reminded of why I occasionally think that brain teasers suck.

First off, a good number of them you will come across when searching are the same ones over and over again. If you like brain teasers like I do, you'll find yourself wishing for something new. If they're not identical, often the idea from a well-know brain teaser is behind the a "new" one. This, I found, is already referenced here in Old Chestnut — and I sympathize.

Next, what some people consider a brain teaser differs from person to person. The first problem in A nice, hard brain teaser isn't a brain teaser to me. It's a story problem, solveable through applying mathematics. This causes problems, as something that is a difficult lateral thinking exercise for someone else might be easy for you. As I am an engineer, mathematics is easy for me. I also indulge in more wordplay than your average engineer.

Third, though I've not seen it on E2, is the problem that people don't always remember the puzzle correctly. The most infamous example, from the rec.puzzle FAQ, would be the "words ending in -gry" problem. Most people don't realize it's supposed to be written "the English language".

Finally, there is the propensity to put the answer too close to the problem, where you might stumble across it. There's a reason I created a new node for answers to A nice, hard brain teaser.

Further Commentary

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