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Subject: [FAQ] rec.puzzles Frequently Asked Questions [weekly]
This article was archived around: Mon, 19 Feb 2001 20:19:43 GMT
Last-modified: Mon Jan 15 2001
Welcome to the rec.puzzles Frequently Asked Questions List. The purpose
of this article is to assist readers in determining if their puzzle is
appropriate for posting to rec.puzzles and to introduce new readers of
rec.puzzles to newsgroup etiquette.
This FAQ is maintained by Matthew Daly and posted weekly. Questions and
comments about this FAQ should be e-mailed to firstname.lastname@example.org or posted
0. INTRODUCTION AND CONTENTS
The rec.puzzles newsgroup is generally friendly, and the signal-to-noise
ratio is quite high compared with many other Usenet newsgroups. However,
many rec.puzzles readers have a MAJOR PET PEEVE -- seeing the same puzzles
(and the same answers, and the same discussions) over and over again.
0. Introduction and contents.
1. What you should do before you post a puzzle.
2. Frequently asked puzzles (specific).
2.1. You have 12 coins, one of which is either light or heavy....
2.2. What are the three common English words that end in -GRY?
2.3. What's the probability that my other child is a girl?
2.4 The Monty Hall Puzzle (Should I switch doors?)
2.5 You come upon a fork in the road....
2.6 Four men are crossing a bridge....
2.7 Albert Einsein's logic puzzle (Who owns the fish?)
3. Frequently asked puzzles (general).
3.1. Lateral Thinking (or Situation) puzzles.
3.2. Sequence puzzles.
3.3. English language records.
3.5. The Equation Analysis Test (26 = L. of the A.)
4. General posting guidelines
4.1 Posting puzzle solutions.
5. Information about the rec.puzzles archive.
5.1. The e-mail archive server.
5.2. Getting to the archive by Internet, FTP, gopher, and WAIS.
6. Information about the rec.puzzles oracle.
7. Credit where credit is due.
1. WHAT YOU SHOULD DO BEFORE YOU POST A PUZZLE:
If the puzzle that you want to post came from a photocopy, or an e-mail
from a friend, it is likely that the puzzle has been in rec.puzzles
before, perhaps even very recently. Before you post any puzzle to
rec.puzzles, you should take the following steps to ensure that the puzzle
is not a chestnut:
o If you haven't read the articles that are regularly posted to the
newsgroup news.announce.newusers, please do so. You will learn
about some general rules and principles that apply to virtually
all newsgroups, like a ban on posting most binary files and MIME
attachments, and sending e-mail instead of posting if your message
is intended only for one person.
o Read the newsgroup for at least a week (Reading for some time before
posting is proper etiquette for ANY Usenet newsgroup. If you didn't
realize that, all the more reason for you to go back and read
o Read the FAQ, with particular focus on sections 2 and 3.
o If you have access to a Usenet archive like http://www.deja.com
check some obvious keywords to see if your puzzle has been discussed
in the past several months.
o Look through the rec.puzzles archive (see Section 5 below).
o Ask the rec.puzzles oracle (see Section 6 below).
If what you really want is not to pose your question for the puzzlement of
rec.puzzles readers, but rather simply to find out the answer to the
puzzle (presumably fairly quickly), you'll generally get a faster
turnaround from the archives and/or the oracle than posting to the
2. FREQUENTLY ASKED PUZZLES (SPECIFIC):
This section contains, short summaries of the most commonly occurring
puzzles on rec.puzzles. Unless you have some devastatingly new twist or
observation on these puzzles, please don't post about it.
Throughout this FAQ, the phrases between arrows ==> like this <== are the
names of puzzles as they appear in the archive. The stuff in [square
brackets] is the name of the file you should look for, once you're in the
rec.puzzles archive directory, if you are accessing the archive by ftp
(see Section 5.2 below). In general, the solutions given in the archive
are much broader than those given here.
2.1. ==> balance <== [logic/part5]
You have 12 coins, one of which is counterfeit. The counterfeit is
indistinguishable from the rest except that it is either heavier or
lighter (but you don't know which). How can you determine which coin is
the counterfeit in 3 weighings on a balance scale?
One solution is to label the coins with the letters from FAKE MIND CLOT
and weigh the coins: MA DO -- LIKE, ME TO -- FIND, FAKE -- COIN. Logic
will now suffice to find the odd coin. For instance, if the results are
left down, balance, and left down, then coin "A" is heavy.
2.2. ==> gry <== [language/part2]
What are the three common English words that end in -GRY?
There are only two: angry and hungry. The rec.puzzles archive offers a
large collection of words that end in -GRY, but none of them could be
considered even remotely common.
There are many generally unsatisfying "trick" answers to the problem,
which depend on a specific wording of the question or that the question be
spoken instead of written. There seems to be no agreement among puzzle
historians about which form is the original, or even the age of the
problem. In any event, it is apparent that the frequent mutations of the
puzzle statement over the years have erased whatever answer was intended
by the original author.
2.3. ==> oldest.girl <== [probability]
If a person has two children, and truthfully answers yes to the question
"Is at least one of your children a girl?", what is the probability that
both children are girls?
The answer is 1/3, assuming that it is equally likely that a child will be
a boy or a girl. Assume that the children are named Pat and Chris: the
three cases are that Pat is a girl and Chris is a boy, Chris is a girl and
Pat is a boy, or both are girls. Since one of those three equally likely
possibilities have two girls, the probability is 1/3.
2.4. ==> monty.hall <== [decision]
You're about to play on a game show. There are three doors; behind one is
a valuable prize, behind the other two, junk. You'll get to choose a
door, and then Monty Hall (who knows where the prize is) will open one of
the other doors, showing you junk. At that point, you'll have a chance to
"switch" your choice to the remaining unopened door. After that, you'll
win whatever is behind the door you have chosen. Should you switch?
It is advantageous to switch: your probability of winning is 2/3 if you do
so. The probability that your first guess is wrong is 2/3, and switching
doors will gain you the prize if and only if your first guess was wrong.
2.5. ==> fork.two.men <== [logic/part4]
Two men stand at a fork in the road. One fork leads to Heaven; the other
fork leads to Hell. One of the men always answers the truth to any yes/no
questions asked of him, the other always lies. Can you find a question
that will allow you to determine the road to Heaven?
One method is to point to one of the paths and to ask either of the men
"Would the other man confirm that this was the path to Heaven?" It is the
path to Heaven if and only if the man answers "no".
2.6. ==> bridge.crossing <== [decision/crossing]
Four men are on one side of a rickety bridge on a dark night. The bridge
is only strong enough to support two men at a time. It is also necessary
for the men crossing the bridge to carry a lantern to guide their way, and
the four men have only one lantern between them. Andy can cross the
bridge in 1 minute, Ben in 2, Charlie in 5, and Dan in ten minutes. How
quickly can all four men be together at the other side?
The solution is surprising to some people because they initially suspect
that it is fastest if Andy escorts everyone across because he can return
the fastest. However, a faster method requires only 17 minutes. First,
Andy and Ben cross (2 min), then Andy returns (1 min). Then, Charlie and
Dan cross (10 min) and Ben returns (2 min). Finally, Andy and Ben recross
(2 min). In short, you save two minutes by having the two slowest people
cross the bridge in the same trip.
"This is a quiz written by Einstein in the last century. He said that 98%
of the world's population could not solve it...." This is followed by a
series of fifteen clues about five European men who live in houses of
different houses, drink different beverages, smoke different cigarettes,
and keep different pets. Who keeps the fish?
The German keeps the fish. A detailed explanation is available at
http://www.frontiernet.net/~mwdaly/recpuzzles/einstein.html. For the
record, Einstein didn't write the puzzle and far more than 2% of the
world's population could solve it.
3. FREQUENTLY ASKED PUZZLES (GENERAL):
This section gives information on some general categories of puzzles that
pop up over and over again.
3.1. ==> situation.puzzles <== [logic/part3 and logic/part4]
A man walks into a bar, asks for a drink, the bartender points a gun at
him, the man says "thank you" and leaves.
The essence of these puzzles is that one person in a group answers a
series of yes/no questions and the other people in the group attempt to
piece together the circumstance that would lead to the posed situation.
Over the years, rec.puzzles has referred to these as "situation puzzles",
although a suite of books by Paul Sloane has made the term "lateral
thinking puzzles" more commonplace.
Because very few of the lateral thinking puzzles posted to rec.puzzles are
original and the structure of Usenet is not conducive to posting and
answering yes/no questions, moderating contests of this sort in
rec.puzzles is a bad idea. Fortunately, Sloane manages a very popular
website where several original puzzles are moderated at a time, at
http://www.lateralpuzzles.com. There is also a very complete list of
puzzles kept by Jed Hartman at http://kith.org/logos/things/sitpuz/.
The answer to the puzzle given above is that the man had the hiccups,
intended to cure them by drinking a glass of water with his nose plugged,
and was glad that the bartender scared him enough to cure him.
3.2. Sequence puzzles
O, T, T, F, F, S, ? ==> series.06 <== [series]
5, 6, 5, 6, 5, 5, 7, 6, ? ==> series.21 <== [series]
11, 21, 1211, 111221, ? ==> series.07 <== [series]
The problem with letter sequences is that we've seen most of them, quite
possibly even that one you just thought of yourself. Check out the
archive, in the general category "series", to make sure yours isn't there.
The problem with number sequences is that there are infinitely many
formulas that will fit any finite sequence, and the concept of the most
"natural" formula is a subjective one. Since number sequences always lead
to this same discussion, it's best to avoid them in rec.puzzles.
A great service for tracking down number sequences is N.J.A. Sloane's
On-Line Encyclopedia of Integer Sequences. Visit them on the Web at
information on the e-mail server, send a blank message to
The next term in the third series is 312211, since the previous term was
"three 1's, two 2's, one 1".
3.3. English language records
What's the shortest sentence with all 26 letters? ==> pangram <==
What's the longest one-syllable word? ==> syllable <==
(all of those are in: [language/part2])
These and many other questions are answered under the general archive
category "language". Keep in mind that everyone has a different concept
of what words comprise the English language, and which words are common.
==> unexpected <== [logic/part5]
Swedish civil defense authorities announced that a civil defense drill
would be held one day the following week, but the actual day would be a
==> envelope <== [logic/part5]
Someone has prepared two envelopes containing money. One contains twice as
much money as the other. You have decided to pick one envelope, but you
can then "prove" that the other envelope contains more money than the one
Threads about these and other logical paradoxes tend to go on for a long
time and can get nasty as people try to convince each other of the truth
of their positions. If you would like to start a thread about a paradox,
please read the archive explanation first to see if that clears things up
for you. Whether you are reading or posting to one of these threads,
remember that there are many logical interpretations that are often
equally valid. If there weren't, it wouldn't be a paradox, would it?
3.5 The Equation Analysis Test
==> equations <== [language/part1]
26 = L of the A (Letters of the Alphabet)
3 = BM(SHTR) (Blind Mice - See How They Run)
27 = NOEBTCGPWTCAAAAPOTBOEOEWEOWTBUAEAAG
The original form of the Equation Analysis Test was first printed in the
May/June 1981 issue of _Games_ Magazine, and has perhaps become the most
photocopied quiz in history. Posting the original quiz (which includes
the first two examples) is unnecessary, as the archive includes all of the
originals plus several hundred extra equations. If you come up with your
own set, it is on-topic to post them, although you want to be sure to
follow-up with your solutions within a week because many of them are hard
to guess with certainty.
4. GENERAL POSTING GUIDELINES
This section describes several specific aspects of the culture of
rec.puzzles that have developed over the years. They are different
enough from the general Usenet "netiquette" that people tend to
wonder what we're doing and why we're doing it.
4.1 Posting puzzle solutions
When someone posts a puzzle that you know the solution to, or better yet,
a puzzle that you discover the solution to after hours of thinking, it's
natural to want to post the solution to proclaim your achievement to the
world. Such posts are called "spoilers". There's nothing wrong with
posting a spoiler, but certain guidelines should be followed.
When you post a spoiler, you should include the warning [SPOILER] after
you quote the puzzle, but before you include your solution. (Some people
also include [SPOILER] in the subject line of their post, although
changing subject lines makes it difficult for some to follow the thread of
a discussion, so it is frowned upon.) After the [SPOILER] flag, insert a
page break by pressing Ctrl-L; it should look like this: ^L. The page
break will cause many newsreaders to pause at that point and give the
reader a chance to move on to the next post if they'd rather wait before
reading your spoiler. If your editor doesn't allow pagebreaks, 24 or more
blank lines will do.
A facet of rec.puzzles culture that comes from the early days of
Usenet is based on the notion that all posts should contain either the
statement, solution, or specific commentary about puzzles. In those
nearly-forgotten days, if you wished to make a generally off-topic post,
you would include an "Obligatory Puzzle" (or ObPuzzle, for short) to amend
for whatever else you were writing about. The practice of posting
ObPuzzles continues, although the necessity for it has disappeared to the
point that posting a frequently-seen puzzle as an ObPuzzle would be
considered more rude than making a simple off-topic post.
One of the entertaining pastimes of rec.puzzles is "sport-flaming", where
the regulars attempt to enliven a puzzle by taking advantage of poor
wording or by simply making it clear that the puzzle poster should have
read the FAQ or checked the archives before posting. If you have been
sport-flamed, please don't take it personally, and PLEASE don't start a
real flame war over it. Nobody is trying to force you from the newsgroup:
it's merely a good-natured way of pointing out that you should have been
more cautious before posting. When you've been around for a couple
months, you'll understand why, and if you've seen netcops in other Usenet
groups, you will probably appreciate our more entertaining style. On the
other hand, if you come to decide that this is too restrictive for your
taste, you might enjoy alt.brain.teasers, which is a less structured
puzzle newsgroup. Also, keep in mind that more ordinary flames, like
other discussion that has nothing to do with puzzles or puzzling, is
tacky and frowned upon.
5. THE rec.puzzles ARCHIVE
The rec.puzzles archive is a treasure trove of puzzles and their
solutions. Maintained by Chris Cole, the archive currently contains over
500 puzzles, including those mentioned in this FAQ, and others of many
varieties. (It is not, as the name might suggest, an archive of all posts
made to rec.puzzles.)
Corrections to and comments on archive entries should be e-mailed to
email@example.com. Discussion of the solutions in the archive
is generally welcomed in rec.puzzles.
5.1. The e-mail archive server
The easiest way to figure out the archive is to get and read the index.
The index contains descriptions of all of the puzzles in the archive and
instructions for receiving individual puzzles. To request a copy of the
index, send e-mail to firstname.lastname@example.org, with a body that looks
like this (assuming you were the President of the United States):
The version of the archive that is available via e-mail is guaranteed to
be the most current.
5.2. Getting to the archive by Internet, FTP, gopher, or WAIS
A keyword search plus links to other puzzle sites
The entire archive is also accessible via anonymous FTP, from any site
which maintains archives of the newsgroups news.answers or rec.answers.
The file part01 contains the index. The remaining files contain
alternating problem text and solution text for all the puzzles.
Some FTP sites are:
>From the global home page, the menu choices to access the archives at
North America/USA/Texas/Texas Tech University, Computer Sciences
To access "uni-hohenheim.de" your menu choices are:
Europe/Germany/University of Hohenheim/Lots of Interesting Stuff
/FAQ Frequently Asked Questions/rec/puzzles/archive
6. THE rec.puzzles ORACLE
This is a group of rec.puzzles regulars, who are familiar with the
rec.puzzles archive, and who will find your answer there if it exists, or
maybe compose an original answer if they are interested enough! At any
rate, they promise to respond to your question within two days, and
perhaps save you the embarrassment of posting a well-worn puzzle. They
will respond even if they do not know the answer to your question.
To query the rec.puzzles oracle, send e-mail containing your question to
email@example.com. Comments and suggestions are always welcome
at the same address.
7. CREDIT WHERE CREDIT IS DUE
The first rec.puzzles FAQ was written by Tom Magliery. Many thanks to him
for all his hard work putting it together. Thanks also to Jonathan Haas,
who began the tradition of posting the FAQ weekly.
Matthew Daly firstname.lastname@example.org http://www.frontiernet.net/~mwdaly/
I am just like any law-abiding tax-paying citizen, except that I
don't obey laws or pay taxes. - Mojo Jojo
Though he is a person to whom things do not happen, perhaps they
may when he is on the other side. -Edward Gorey