NEWS

• 7:58 PM 8/29/2009
  The server at which the forum is hosted, OCF.berkeley.edu, was down for the last month. Services have been partially restored as of last week, but the uptime has been erratic ever since. The latest news about when stable service can be expected for the forum can be found at http://ocfblog.blogspot.com.
  
  
• 8:46 PM 8/19/2004
  New puzzles recently added:
  
  Dominant Fifth
  Cute Permutation Inequality
  
  Hexagon In Circle
  Elegantly Greedy Pirates
  Fair Coin Gambling
  Jumping Frog On Ladder
  Coloring The Rational Plane
  Random Points On Sphere
  Number of Midpoints
  
  The Mirrored Maze
  Typewriter Gibberish
  Three Dimensional Random Walk
  Temperature Antipodes
  Convex Set Projection
  Filling A Box With Cubes
  10-Adic Numbers
  DeBruijn Sequence
  Guessing Cards Suit
  Prolong the Line
  Waiting For Patterns
  
  Invertibility of I-AB
  
  
• 4/7/2003 1:18AM
  

  For the latest puzzles, please frequent the forum as a regular member. The cover page will only be updated once in a while. New problems arise in the forums before they make it here. Plus you'll get to meet some very witty people.

  Many thanks to the kind forum regulars who have been guiding newbies over the past few months.

• 3/9/2003 1:36AM
  There has been interest in an online newsletter. Would you like to be an author? Click here if you are interested.
  
  
• 12/20/2002 8:28PM
  Term paper on the riddle forum as a model online community! Select members were interviewed via phone. Author: Carl Cox, Georgia Tech CS grad. student.
  
  
• 10/15/2002 2:10AM
  HEY! Who are you?
  How did you find the site?
  Please, introduce yourself!
  
  

INTRODUCTION

WELCOME to [wu:riddles]! This is an archive of problems I've been collecting since the Spring of 2002. They come from many places, including word of mouth, college courses, books, and job interviews for hi-tech positions. Many are even written by members of our own forum community. These carefully chosen puzzles will demand you to think in creative ways you perhaps normally would not. In fact, some will seem outright impossible at first ... but once you crack them, the epiphany can be truly rapturous!

If you are a first-timer, here are some problems I recommend you start out with:

The riddles are organized by difficulty as judged by myself: easy, medium, and hard. Then there is the microsoft section, consisting of weird, open-ended consulting-style questions. The cs and putnam sections contain problems which may require a particularly specialized background in computer science and mathematics, respectively. Later, I will add sections for "What Am I" and "What Happened" riddles.

Hints are available for some riddles. To see a hint, highlight the area near the word "Hint:" with your mouse. This mouse-highlighting convention is used in both the forums and this cover site.

If you need solutions or more hints, please post to the FORUM! Almost all puzzles are solved in great detail there. Also, if you want to contribute a new puzzle, please post to the FORUM as well. We have a daily healthy discussion of new problems, and every once in a while I will export some of those problems to the cover site. The forum is a little slow, and I apologize for this, but please be patient. Alternatively, you can send me an e-mail directly. (Remove the "NO_SPAM_" prefix.) However, my response time may be very slow, as I'm often busy these days; the forum is definitely preferred.

My favorite kind of puzzle satisfies the following criteria: 1) It seems impossible, 2) Most people can understand it, and 3) It does not require specialized knowledge to solve. When I started collecting, I tried hard to find problems that satisfied these things. However, as the site has grown over the years, it has become apparent that for many of the harder problems, you won't have a chance without knowing some post-high-school mathematics. Basic probability and combinatorics are particularly common. If you lack this background, please do not be discouraged. Our friendly and active forum community can help you get up to speed. My sincere hope is that you will leave with a greater appreciation of the power of math.

Many thanks to all the cool people who have sent me new material. Also, I express much gratitude to towr and Icarus for their extensive work in generously maintaining the forums. I also thank David Lau, Eric Yeh, and James Fingas for contributing an exceptional number of problems when the site was getting started. Let's make this the definitive riddle archive.

Finally, I would like to again plug the forum, which i invite you to browse and perhaps even join. You will be very hard-pressed to find another problem-solving community of comparable quality. It is the neatest thing on this site, a giant board of discussions for thousands of puzzles. A LaTeXish syntax is also provided to support writing of mathematical notation. New problems arise almost every hour, and we have members all over the globe, at all ages, from professions ranging from graphic designers to engineering professors. Having the privilege of building such a community is one of the nicest things I've ever taken part of.

Whatever your reasons for being here -- whether you are a life-long mathemagician, or a desperate kid googling for tricky homework solutions -- I hope you will stick with this site regularly, and come to appreciate the joys of problem solving.

- William Wu, 4/2/2002 11:51PM; Revisions: 9/29/2002 7:24PM; 11:40 PM 8/19/2004, 12:55 PM 2/5/2005

SYMBOLS

FORUM

RIDDLE INDEX