November 2015 
How to Make a Torus By Laszlo C. Bardos
Constructing the MiyaotoMoneroaMonterde model of a torus: printable template and video instructions.
Constructing a paper torus with seams along Villarceau circles: printable template and video instructions.

April 2015 
The Intersection Game by Burkard Polster
ZenZen by Thomas Q. Sibley
ZenZen Puzzles
ZenZen Solutions

February 2015 
Fibonacci or Fairy Tale? by Burkard Polster and Marty Ross
Supplementary material
Letter to Joel by David and Joel Stucki
Supplementary material
Epic Math Battles: Counting vs. Matching, by Jennifer J. Quinn
"Who presented the most beautiful proof of Binet's formula? The Countess or Sir MatchALot? Vote now!"(Voting closes on March 21, 2015)

November 2014

Reverse Engineering College Rankings by Tim Chartier and Justin Peachey
Supplementary material
Cryptographic Word Search by Tom Edgar and Andrew Lloyd
Supplementary material
AFTERMATH: When Will I Use This? by Douglas Corey
"When Will I Ever Use This? An Essay for Students Who Have Ever Asked This Question in Math Class"

April 2014

TracTricks by Burkard Polster
Here is the template used to make the cones for the paper pseudosphere.
X Goes First: Wild Tales of a TicTacToe Grandmaster by Bryan Clair
Here are the 765 different tictactoe boards and the paths through them.
BovinoWeierstrass and Other Fractured Theorems by Matt Koetz, Heather A. Lewis, and Mark McKinzie
Enjoy more fractured theorems and the "cheat sheet" that goes along with all of them.

February 2014

A Conversation with Steven Strogatz by Patrick Honner
A continuation of the conversation can be found here.

November 2013

Fasten Your Seat Belt and Start Your Engine by Charles R. Hadlock
The advertised simulation can be found here.
A Magic Trick Based on the Hamming Code by Todd Mateer
Printable cards (pdf) and Mathematica source code is available here.

September 2013

Budapest Semesters in Mathematics: A Study Abroad Utopia by Gregory Michel
The solution to "ThreeHeaded Dragon" can be found here.
Beading the SevenColor Map Theorem by Susan Goldstine, Sophie Sommer, and Ellie Baker

September 2012

270 by Chuck Wessell
Election data can be found here.

November 2010

"O, IC!" Crossword by Sam Vandervelde
The solved crossword is availble here.

September 2010

The Playground  by Derek Smith
For problem 241: Download a Mathematica notebook that provides a solution. Mathematica (version 7 or higher) is required.

April 2010

The Playground  by Derek Smith
Here is the solution to Andy Liu's cake cutting problem posed in the November, 2009 issue.

February 2010

Change for a Dollar, Change for a Million  by Martin Erickson.
This Mathematica notebook file contains all the computations performed in the article, including complete solutions to the exercises. The file ChangeForADollar.nb requires Mathematica (version 7 or higher).
The Playground  by Derek Smith
For problem 242: Download a Mathematica notebook that will allow you to flip a stack of pancakes to your heart's content. Mathematica (version 7 or higher) is required.

November 2009

Soggy Jogging  by Dank Hailman and Bruce Torrents.
This demonstration allows you to dynamically control both weather conditions and the size and shape of a 3dimensional traveler as he runs through the rain. There are two versions. The file SoggyJoggingDemo.nb requires Mathematica (version 6 or higher). The file SoggyJoggingDemo.nbp requires the free Mathematica Player application, which is available here.
The Playground  by Derek Smith
For problem 230: Download a Mathematica notebook that will allow you to rotate and adjust the parameters for a virtual sculpture.

April 2009

Four Coloring the US Counties, by Mark McClure and Stan Wagon
Lyrics to "A Four Color Proof," by Jason Holt
The Playground edited by Derek Smith
Problem 220. InscribeAGon ended with an open question. A discussion can be found here.
Problem 221. Cutting Cake is still open. Andy Liu offers the following hint: If Alice's first cut produces two pieces of sizes 20/53 and 33/53, how should Brian respond?
Problem 229. Colorful States asks you to fourcolor a map. Download a nice fullpage pdf version of the map here.
MATHORIZN Sudoku puzzle. Here is the solution.

November 2008

A Dozen Questions about Pascal's Triangle by James Tanton
Eagle eyed readers comment on some nonmathematical content. Letters are posted from Leon Harkelroad and Deborah Gray plus a short response from the author.
Perspectives on Mathematics in Art History by James Galloway
Letter to the editors from Annalisa Crannell correcting our perspective on perspective.

September 2008

Problems and Solutions edited by Andy Liu and Derek Smith
Problem 213. Five Cubed remains open. The extensive hint promised by an editor is provided for you here. Readers are still encouraged to submit their determinations on how many different solutions are possible.

April 2008

The Most Marvelous Theorem in Mathematics by Dan Kalman
This publication of this article purposefully coincided with two other pieces penned by the same author. Both are worth your time and energy.
The Most Marvelous Theorem in Mathematics. Journal of Online Mathematics and its Applications, April 2008.
An Elementary Proof of Marden's Theorem. American Mathematical Monthly, Volume 115, Number 4, April 2008, pp 330  338.

September 2007

Taking Sudoko Seriously by Laura Taalman
This article had many wonderful puzzle variations on Sudoku (Figures 914). The solutions are available. 
April 2007

Just in case you weren't sure, YES this is an April Fool's Issue.
Mathematical Enquirer (Volume 1 Issue 0, April 1, 2007)
One piece prompted the following response:
Doron Zeilberger's Opinion 80: Shalosh B. Ekhad's Reply to the Math Horizons "April Fools" Spoof Where It Was Extensively Quoted (or Rather Misquoted)

November 2006

Number Theory Crossword Puzzle by Thomas P. Dence and Jenise A. Smalley
The puzzle is currently available. Solutions are finally available.

September 2006

Pentominoko by Laura Taalman. Solution available.
Hidden Word Crossword and Contest by Laura Taalman.
Tshirts go to David Goldsmith, faculty member at the College of Saint Rose, David Rhee, student at McNally High School, Ross Brater, undergraduate at Ohio Wesleyan University, and Lola Thompson, undergraduate at University of Chicago for submitting the correct answer and a number closest to the randomly selected number 83. The original crossword and its solution are available. 
April 2006

Selfanswering Question ContestWinning entries from September 2005 contest. Plus the Mathematica code to verify entry by Tupper and Wagon and even more selfanswering questions for your enjoyment. 
February 2006

Links to some of our artistic authors.
Hidden Word Crossword and Contest by Laura Taalman.
Submit your solution for fabulous prizes by April 15, 2006.

November 2005

A (Mostly) Differential Equations Crossword Puzzle by Thomas Dence, Jenise Smalley, and Kenneth Gasser
The puzzle and solution are available.
Thanks to everyone who participated in the online Math Horizons survey. Special congratulations go to senior mathematics major Brian Worthen from Bryan, Texas. He was randomly selected from all the participants to receive five free MAA books!

September 2005

Mathematical InversionsWinning entries from November 2004 contest.
Liars and Truthtellers: Learning Logic from Raymond Smullyan by Robert L. Holliday
Additional puzzles and solutions for your enjoyment.
The Virginia Tech Regional Math Contest by Peter A. Linnell
The question from a previous VTRMC and its solution are available.
Selfanswering Question Contest by Chris Hill
Submit your entry (both question and answer!)

April 2005

Playing with Matches by Eric Libicki
Questions and answers available.

February 2005

A reader response to Stuart Boersma's article A Mathematician's Look at Foucault's Pendulum. It gives a simpler geometric derivation of the same result and would be suitable for discussion in an undergraduate or even high school math class. The result is one of those delightful "AHA!"s that make mathematics so much fun.
A Comment on Stuart Boersma's article "A Mathematician's Look at Foucault's Pendulum" by Greg Johnson (posted June 2005)
SelfReferential Aptitude Test by Jim Propp
Questions and answers available.

September 2004

Biography of a Contest Problem by Steven Dunbar & Kevin Hankin
Solution to regular npointed star problem.

April 2004

Analysis Word Search by Thomas Dence, Ken Gasser, Jenn Meyer, & Jenise Smalley
The puzzle and solutions are available.

November 2002

A reader response to The Devil is in the Culture by sarahmaria belcastro with Aaron Howard. It gives a different perspective on Enzensberger's The Number Devil and provides some insight on the original German.
"Personal Refections on The Number Devil" by Angie ThuyAnh Mai (posted June 2005)

November 2000

The Final Exam: World Wide Web Treasure Hunt — solutions and winners! 
April 1999

The Final Exam: Pi Mnemonics — winning entry! 
February 1999

The Final Exam: Who Said It? — solutions. 