Fortnightly Topic Challenges are currently on hold, due to the low activity that was unfortunately seen in the last few installments. They will be continued once enough popular suggestions are present again (or once enough users want FTCs to restart again). In the meanwhile, you can still vote on and suggest topics here, as they will still be used for the eventual restart.

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:

  • March 21st – April 3rd, 2021
    Polyominoes suggested by Bubbler

Previous Topics:

  • March 7th – March 20th, 2021
    Non-rectangular grids suggested by Bubbler
  • February 21st – March 6th, 2021
    Escape Rooms suggested by Stiv
  • February 7th – February 20th, 2021
    Pub Quiz Camouflage suggested by Stiv
  • January 24th – February 6th, 2021
    Unusual tag mix suggested by melfnt
  • 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 $\endgroup$ Commented Nov 25, 2020 at 15:42
  • 1
    $\begingroup$ @BeastlyGerbil That might cause irregular voting from users who just want to keep the series alive, or even the opposite. $\endgroup$ Commented Feb 25, 2021 at 23:02

13 Answers 13


- Current Topic -


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!



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.


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
    Commented Dec 23, 2020 at 8:12

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
    Commented Oct 31, 2020 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 $\endgroup$ Commented Oct 31, 2020 at 18:43
  • $\begingroup$ @AnthonyIngram-Westover That why it's a challenge :) $\endgroup$ Commented Oct 31, 2020 at 18:56
  • $\begingroup$ @bobble Does this answer your question? puzzling.meta.stackexchange.com/questions/7023/… $\endgroup$ Commented Oct 31, 2020 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
    Commented Oct 31, 2020 at 18:59
  • $\begingroup$ @bobble This is the only puzzle tagged computer-science that isn't mine that even mentions python. $\endgroup$ Commented Oct 31, 2020 at 19:03
  • $\begingroup$ Also, that puzzle is not bad! $\endgroup$ Commented Oct 31, 2020 at 19:04

Game/Quiz Shows

This... is... Puzzling! Let's meet this fortnight's topic challengers...

Create a puzzle themed around a real-life game or quiz show. The puzzle must involve the basic format of the overall show in some way, e.g. a Jeopardy!-themed puzzle might include answering trivia questions chosen from a board, while an Only Connect-themed puzzle might include questions and a .

Some examples on this site, including the game/quiz shows they reference:

And Wikipedia has "Lists of game shows" from a variety of countries, linked here for informational and inspirational purposes.


  • Puzzles that are dressed up as or reference game/quiz shows but do not directly involve the format of a real-life show are not eligible for this challenge, e.g. the Monty Hall problem, a standalone
  • Puzzles asking about optimal strategies for playing game/quiz shows are eligible, as long as what the "best" strategy is can be objectively quantified, e.g. Monopoly Game Show: Is there a winning strategy?
  • $\begingroup$ Let me know if any of the requirements are confusing - I'll admit, some of these rules and examples may be a bit contradictory, but I'm not sure how to clearly state my intent with this proposal yet $\endgroup$
    – HTM
    Commented Mar 12, 2021 at 2:09

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!


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.


Sinister Stories in Casual Concepts

A puzzle, most likely tagged with or , where the theme is dark and sinister, and it's the solver's task to find out what common object or situation the puzzle is hinting at.

Some great examples are:

And an example I made:


Puzzles about Stack Exchange

There are a few puzzles which topic regards the scoring mechanism, the privilege system, the users, the ids of the posts or other features of stack exchange. Here are some of my favorites:

My proposal for this challenge is to create more puzzles like these. They can be about Puzzling Stack Exchange or Stack Exchange in general. Please read this before creating such puzzles: Riddles involving Stack Exchange: Off topic?

  • 1
    $\begingroup$ I'm not sure this is a good idea - it seems like it will lend itself only to using information that may change over time, which doesn't make for a long-lasting puzzle. (And if you refer to specific users, apart from the problem of "the users may leave, making the puzzle unsolvable" there's also the fact that many people don't like having their names used in puzzles.) $\endgroup$
    – Deusovi Mod
    Commented Feb 16, 2021 at 1:42
  • $\begingroup$ (Echoing Deusovi's comment: we have guidelines about using people's names in puzzles)) $\endgroup$
    – bobble
    Commented Feb 16, 2021 at 1:43
  • 1
    $\begingroup$ @Deusovi I linked a meta question which has an answer in which you state that "Questions involving Stack Exchange are fine, but you should take care to note that they're solvable long after they're posted". I agree with your answer and I don't see what's wrong in posting a puzzle that fit that guidelines $\endgroup$
    – melfnt
    Commented Feb 16, 2021 at 18:47
  • $\begingroup$ @bobble I know, I wasn't referring to posting puzzles with someone's name on them. Also, many of these puzzles are not about SE, they just use the usernnames $\endgroup$
    – melfnt
    Commented Feb 16, 2021 at 18:48
  • $\begingroup$ Will a puzzle that mentions PSE users as Among Us players count? $\endgroup$ Commented Mar 2, 2021 at 4:15
  • 1
    $\begingroup$ @Anonymus25-ReinstateMonica no. I want you to create puzzles about the stackexchange mechanisms, not just to use the usernames in an arbitrary puzzle. Also, you should be careful when using the other people's username in a puzzle: see bobble's answer above. $\endgroup$
    – melfnt
    Commented Mar 3, 2021 at 9:05

A previous Puzzle (part 2)

A puzzle based on a previous puzzle posted on PSE by another user and that is not already part of a series: it could be the same game with a slight variation in the rules, it could be the same puzzle with a different schema or just a continuation of the story in the previous puzzle.

There are many examples of this kind of puzzles, but they are a bit difficult to find. The only one I can think of is Prime Number Snake, posted by @BernardoRecamánSantos and Prime number snake (2), posted by @daw.


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!


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