Let's do fortnightly topic challenges once again!

What to Do:

If you have an idea for a tag or a theme of any kind to use as a topic challenge, post it as an answer below. (Do note, you can propose anything, not only a tag). Only one suggestion per answer, please. Here is a list of all tags, to help.

At the start of each fortnight, the highest-voted answer to this post will be selected as that fortnight's topic. Starting from today, users can propose their themes or topics. The selected answer will then be deleted to reduce clutter in the list.

After the selection, a new question will be created in this format. An answer will then be posted to that question with links to all the posts in the featured topic in the fortnight.

We'll again keep a list of all the topics.

Happy Puzzling!

Current Topic:

  • January 24th – February 6th, 2021
    Unusual tag mix suggested by melfnt

Previous Topics:

  • January 10th – January 23rd, 2021
    Wacky Sudokus suggested by Beastly Gerbil
  • December 27th – January 9th, 2021
    "Tales From the Cryptic" suggested by Stiv
  • December 13th – December 26th, 2020
    Flags suggested by Stiv
  • November 29th – December 12th, 2020
    Introduce a new grid deduction genre to the community suggested by Bubbler
  • November 15th – November 28th, 2020
    Variety Crossword Grids suggested by bobble
  • November 1st – November 14th, 2020
    Wordless Connecting Walls suggested by Stiv
  • 4
    $\begingroup$ A proposal: If/when all remaining answers don't have 5 or more upvotes, then that should be taken as a sign that proposals have run out and the series should (temporarily) stop once more $\endgroup$ – Beastly Gerbil Nov 25 '20 at 15:42

15 Answers 15


- Current Topic -

Unusual tag mix

I like puzzles that mix very different topics and solution methods, even completely unrelated to each other. So my proposal for this challenge is simple: pick any two (or more) tags that are unlikely to be seen together and create a puzzle.

Some of my favorite mixes:

To help you with choosing the tags, I set up this query on SEDE that selects the unused pairs of tags considering only the top 30 tags on PSE (i.e. for each line of the query results there is no question tagged with both and ). Here is a snapshot of the results as of 2020-12-08; re-run the query at any time to check if something changed:

tag1 tag2
calculation-puzzle cryptic-clues
calculation-puzzle geography
calculation-puzzle rebus
calculation-puzzle steganography
chess computer-puzzle
chess english
chess language
chess pattern
chess word-property
combinatorics cipher
combinatorics cryptic-clues
combinatorics english
combinatorics geography
combinatorics language
combinatorics rebus
combinatorics rhyme
combinatorics steganography
combinatorics word-property
cryptic-clues word-property
geometry cryptic-clues
geometry english
geometry language
geometry rebus
geometry wordplay
optimization cipher
optimization cryptic-clues
optimization geography
optimization language
optimization rebus
optimization rhyme
optimization steganography
optimization wordplay
optimization word-property
rebus word-property
rhyme word-property
riddle optimization
strategy cryptic-clues
strategy english
strategy language
strategy rebus
strategy rhyme
strategy riddle
strategy steganography
strategy word
strategy wordplay
strategy word-property

If you want you can run the same query to get the unused pairs considering every tag; just remove or comment out lines 5 and 6 and be patient: the query takes some 10-15 minutes to execute. Here is the complete list of unused pairs of tags as of 2020-12-08.

I'm curious to see what the community will come up with!


Pub Quiz Camouflage

The pub quiz is a staple form of entertainment in British culture, usually involving teams of patrons in a pub or bar competing against each other to answer trivia questions. Sometimes the rounds have clever hidden themes for which bonus points can be awarded (e.g. spot the connection between all of the answers...).

This challenge would use the concept of a pub quiz as its foundation, involving a set of questions (with allowance for even more specific trivia themes on this site like , or , for example). However, the twist is that there is another sub-puzzle to the quiz, dependent either on its answers, its layout/presentation, or the wording of its questions. However it is concealed, there is more to this quiz than meets the eye!

A straight-out quiz would not be the style of Puzzling.SE - but a quiz with a puzzly twist?? Surely!


Escape Rooms

In (what I am here terming) an 'Escape Room' puzzle, the solver is presented with several seemingly disparate pieces of information in the form of sub-puzzles whose interconnections may not immediately be apparent at first glance. Usually the aim of the overall puzzle is to solve each sub-puzzle, finding their interconnections along the way, and ultimately identify a specific piece of information that enables the scenario to be 'escaped' - e.g. a passcode to pass through a door, or a location where somebody or something might be found.

Crucially, all sub-puzzles are necessary (or at least provide additional hints) for finding the end solution - red herrings are rarely welcome, and extraneous sub-puzzles with no apparent bearing on the solution path are a waste of time for which the setter would not be thanked.

There have been several hugely impressive 'escape room' type puzzles on PSE in the past, perhaps most notably:

As the mammoth upvote counts on these examples show, when this type of puzzle is done well it tends to be incredibly well-received and appreciated by the community (people love these!) - however, recently we have seen new Escape Room puzzles only very rarely. While I appreciate that this is partly because such a puzzle often requires a large investment of time and imagination to prepare and create, making this an FTC topic would present the perfect opportunity for PSE-ers to show their ingenuity and try something new and exciting - and there is nothing to stop puzzle setters getting started on creating their Escape Room puzzle ahead of time to get a headstart on the fortnight!


Used as Fortnightly Topic Challenge #47

Wacky Sudokus

In case you can't tell, weird variants are a personal favourite of mine. They have both been very enjoyable to make, but also rewarding in seeing the feedback and interest received.

I know a lot of people on this site are fans of the youtube channel Cracking the Cryptic, where many such wacky sudokus have been seen before

Wacky sudokus are sudokus that use rules or concepts that have yet to be seen before, or are very uncommon. They can pretty much involve anything, as long as it is not a normal sudoku.

Here are some examples of such wacky sudokus:

I think this would be a fun topic to see what the community can come up with!



often goes with other tags, such as

but may also be a successful puzzle by itself, as in Polyominoes to construct alphabet.

Polyomino is a long-time subject of recreational mathematics, and its properties can be used for grid deductions in many creative ways. So the challenge is to create a puzzle related to polyominoes in a creative way. Of course it does not need to be polyominoes made of squares; it can be polyiamonds (made of triangles) or polyhexes (made of hexagons) too!


Cellular Automata

Here's a regular tag that doesn't seem to get much love:

This can involve puzzles about a well-known cellular automaton, like for example Conway's Game of Life, or a cellular automaton you've created on your own!

Get creative! Maybe we need to reverse-engineer something, try to produce a certain output with given starting conditions, or something entirely different! Crossovers are of course allowed and encouraged.

Great examples of puzzles on this site are Game of Life: Kill the Sun, Can you recreate this fractal I randomly made? and Checkerboard Infection

  • $\begingroup$ I want to point out that Wireworld is also an interesting cellular automaton (awesome examples). Given that Kill the Sun is a "complete this task in GoL" kind of puzzle, "complete this task in Wireworld" should work too. $\endgroup$ – Bubbler Dec 23 '20 at 8:12


Create a puzzle where the first step is a whose solution reveals further steps that must be taken. Crucially, the full puzzle is entirely self-contained within the nonogram and flavour text. (i.e. Not just using a nonogram as the first step in a long string of grid-deduction puzzles which require additional grids to be displayed in the post.)

Good examples from the past include these two puzzles by @jafe, one of which conceals a and the other a set of puzzles.


Non-rectangular grids

Create a puzzle that acts on any kind of non-rectangular grids. The grid can be simple triangular or hexagonal grids, but be creative: there are so many different plane tilings using regular polygons, and you can even use non-regular ones!

Excellent examples include Octagonal Kurotto and Karst - a variant of Cave. Cryptic hourglass is also a notable example of (essentially hexagonal) grid applied to a crossword in a non-traditional way (the words can bend in the middle).

Personally inspired by snub square tiling, whose dual circle packing has an interesting property that all circles touch with exactly five other circles:


Möbius Strips, Klein Bottles, and other unusual topological surfaces

enter image description here

Imagine an elastic square as above. If you ignore the red edges and glue the blue edges so that they have the same orientation, you get a Möbius Strip. If you join both pairs of edges, you get a Klein Bottle. This is not the only interesting surface; you can have 12 different topologies in total by joining two pairs of edges of a square, which include:

enter image description here enter image description here enter image description here

(The last one is a plain torus.) The universal rule is that, if you're on the surface and you walk through an edge, you enter back to the grid through the matching edge with same orientation.

For example, consider this 4x4 grid:

A1 A2 A3 A4
B1 B2 B3 B4
C1 C2 C3 C4
D1 D2 D3 D4

Assume the Klein bottle topology at the top. If you're at B1 and walk left, you end up at B4 (by going through the red parallel edge). If you're at A2 and walk upwards, you end up at D3 (by going through the blue twisted edge).

For the first one on the second image set (the one where the matching edges are adjacent to each other), if you walk upwards from A1, you enter back at A1 but facing right!

The challenge is to create a puzzle that involves an unusual topological surface (which excludes plain wrap-around mechanic a.k.a. cylinder and torus). Such a puzzle may involve tiling, graph, crossword, or a grid-deduction genre, among others.

Some good examples:

You can search for puzzles containing torus, toroidal, or wrap around for some ideas.

There is even a programming language called Klein that can run the same code on different topologies!


Programming Puzzles

A programming puzzle most likely uses the tag and/or (weird flex, but okay).

Programming is my passion, but sadly I'm not seeing much (if any) programming puzzles lately, python programming puzzles to be specific. Yet most of the posts I do see tagged with don't involve actual coding.

This challenge is to create a good puzzle that requires a bit more coding and a little less story-riddle stuff. Stack Overflow is the biggest site on Stack Exchange, and it would be nice to be more welcoming to those users.

  • $\begingroup$ Could you provide an example of what you would consider a "good computer-science puzzle that requires a bit more coding and a little less story-riddle stuff"? $\endgroup$ – bobble Oct 31 '20 at 18:35
  • $\begingroup$ I generally agree, because I love programming and the idea of programming puzzles, but I feel they are really hard to properly pull off. Programing is more art than science, and there's pretty much never just one single answer. It's also really restrictive; the number of people who can reasonably solve a computer programming puzzle is really limited $\endgroup$ – Anthony Ingram-Westover Oct 31 '20 at 18:43
  • $\begingroup$ @AnthonyIngram-Westover That why it's a challenge :) $\endgroup$ – risky mysteries Oct 31 '20 at 18:56
  • $\begingroup$ @bobble Does this answer your question? puzzling.meta.stackexchange.com/questions/7023/… $\endgroup$ – risky mysteries Oct 31 '20 at 18:57
  • $\begingroup$ No, I have seen that, I would like a link to a main-site question you feel is a good one. $\endgroup$ – bobble Oct 31 '20 at 18:59
  • $\begingroup$ @bobble This is the only puzzle tagged computer-science that isn't mine that even mentions python. $\endgroup$ – risky mysteries Oct 31 '20 at 19:03
  • $\begingroup$ Also, that puzzle is not bad! $\endgroup$ – risky mysteries Oct 31 '20 at 19:04

Logic Around the World

Quite simply, use a combination of the tag and the tag. This puzzle and this one are great examples. The only rules are that they must contain BOTH of these tags!


The Puzzles Around Us

Sometimes you are wondering: why the keyboard of your friend is very strange, how to change your baby's shirt while they're still drinking their milk, or what is the best strategy to go to the campus while also refilling your bottle.

Undoubtedly as human, we think and reason a lot. Tackling -life problems will be more fun and fulfilling if we consider them as puzzles!


Twisted Chess

I've seen many chess puzzles where it is a variant of chess, and not about chess itself. So, the goal of this challenge is to do just that! It can be a new piece, a new variant, an existing variant, anything! (Just not using the rules of regular chess.) The puzzle goal can be a mate in 1, a winning position, or an optimization puzzle. Your only limit is your imagination!



I enjoy reading the solutions to metapuzzles, they always are interesting. Deusovi's old Chess Fortnight puzzle is one great example. Metapuzzles could be stuffed into one question, or spread out in a series with a final puzzle that ties all the previous ones together.


Let's go, Zendo!

This is not about the Zendo chatroom game. I'm talking of puzzles where the goal is to guess a secret rule like Zendo. The What Is A Word/Phrase series is an example. But instead of just phrases or words, you can use numbers, grids, or any kind of string. Words or phrases are of course allowed. For an example, Lukas Rotter's Awry Grid puzzle is something to consider.


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