Usenet Newsgroups : rec.puzzles FAQ
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. Questions and comments about this FAQ should be e-mailed to email@example.com or posted to rec.puzzles.
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. 3.4. Paradoxes. 3.5. The Equation Analysis Test (26 = L. of the A.) 4. General posting guidelines 4.1 Posting puzzle solutions. 4.2 ObPuzzles 4.3 Sport-flaming 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 news.announce.newusers.) 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 newsgroup. ---------------------------------------------------------------------- 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. 2.7 "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 http://www.research.att.com/~njas/sequences/eisonline.html. For information on the e-mail server, send a blank message to firstname.lastname@example.org. 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. 3.4. Paradoxes ==> 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 surprise.... ==> 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 you chose. 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. 4.2 ObPuzzles 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. 4.3 Sport-flaming 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): return_address email@example.com send index 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 Internet http://ce.et.tudelft.nl/~arlet/puzzles/ Partially HTMLized. http://alabanza.com/kabacoff/Inter-Links/puzzles.html http://xraysgi.ims.uconn.edu A keyword search plus links to other puzzle sites FTP 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: North America: ftp://rtfm.mit.edu/pub/usenet/news.answers/puzzles/ ftp://ftp.uu.net/usenet/news.answers/puzzles/archive ftp://mirrors.aol.com/pub/rtfm/usenet/news.answers/puzzles/ ftp://ftp.cis.ksu.edu/pub/mirrors/news.answers/puzzles/ Europe: ftp://ftp.cs.ruu.nl/pub/NEWS.ANSWERS/puzzles/ ftp://src/doc.ic.ac.uk/usenet/news-faqs/news.answers/puzzles/archive ftp://uni-paderborn.de/doc/FAQ/rec/ Asia: ftp://ftp.hk.super.net/mirror/faqs/puzzles/ GOPHER >From the global home page, the menu choices to access the archives at "cs.ttu.edu" are: North America/USA/Texas/Texas Tech University, Computer Sciences /Entertainment/Games/Puzzles 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 WAIS wais://xraysgi.ims.uconn.edu:8000/rpa ---------------------------------------------------------------------- 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 firstname.lastname@example.org. 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 email@example.com http://www.frontiernet.net/~mwdaly/