Are You Sure You Know The Rules Of Sudoku?

*** TODAY'S PUZZLE ***
Tonight's sudoku would have some claim to be the most innovative puzzle of all time. We won't tell you why - just solve it and you'll see. It's by "Karl The Fog", which we understand to be a pseudonym for ThePedallingPianist and Michael Lefkowitz. 3 stars out of 5 for difficulty and a 100% approval rating...

Play the puzzle at the link below:

Rules:
Oh! You're solving my puzzle, aren't you? That's so exciting! Oh, right, the rules... what were they... Okay, yes, here we go: Normal 6x6 Sudoku rules apply (obviously); Red lines are "Modular" lines - every 3-cell sequence on a red line contains a digit from {1,4}, {2,5} and {3,6}; Blue lines are "Region Sum" lines - box borders divide them into multiple segments with the same sum; Lines only move orthogonally; Lines can't share cells with other lines; Red lines are "Modular" lines -- wait, did I do that one already? I can't remember; Cages are uh.... hmm... oh yeah, "Killer Cages!" - Digits in them can't repeat, and their sum is always always always given in the Upper Left corner (the leftmost cell of the topmost row of the cage) - And they don't overlap; I'm covering most of the grid, but if you place a *correct* digit, I'll clear from that cell and the cells around it.
Whew! I'm sure those were all the rules. Oh, I'm so hopeful that you can solve it!
-- Karl the Fog

Thanks to all of you who have helped us achieve the milestone of 600,000 subscribers on YouTube. Below you can find a link to a free puzzle pack we have curated to celebrate :)

*** NEW SUDOKU HUNT BY Scojojobo JUST RELEASED ***
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There's also a new solve of Jay Dyer's brutal sudoku: Repeat Prescription

Other treats on Patreon include:

- The Lord Of The Rings Sudoku Hunt by Blobz (and Simon's solve videos)
- Simon's solve of Sky-Skyscraper by Phistomefel has just been uploaded for our patrons.
- Mark's latest solve of The Times Club Monthly cryptic crossword
- Mark's latest series of solves of Connections, the daily New York Times puzzle based on the Only Connect wall game.

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▶ Contents Of This Video ◀

0:00 Theme music & puzzle intro
1:38 American Airlines update
2:43 New App Next Week!
3:04 New Sudoku Competition etc
3:28 Happy Birthdays etc
6:46 Rules
12:12 Start of Solve: Let's Get Cracking

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[ThePedallingPianist]

Thank you so much for such a brilliant and incredibly punctual feature of this one! Juggler is a creative genius and a lovely person and it was a real pleasure setting with him! Shout outs also need to go to my dad (bellal), Justin V (PuzzlePusher), and ViKingPrime for very thorough testing - which, as you can imagine, this puzzle needed!

(spoilers, and indeed a bit of a dissertation(!) for those interested in how it was set, below)

The concept was inspired by some of the recent gdc puzzles on the channel, which involve being able to extend seemingly unremarkable lines surprisingly deep into the fog, resulting in a digit being found somewhere very unexpected. Those deductions in themselves are absolutely breath-taking and I admire gdc enormously for designing them so beautifully, although they leave the setter with a problem.

The thing that makes a deduction like that (and many others in any puzzle, especially Fog of War) so stunning and enjoyable for the solver is the fact that it's so surprising, but now, the really long line that the solver proved had to exist under the fog is very powerful in leading to many further logical deductions, none of which are surprising any more because the solver already proved that the line had to exist.

So then it occurred to me that a beautiful logical step like that doesn't rely on the really long line actually existing under the fog, it relies on the solver *believing* that the really long line must exist under the fog - there's a subtle difference, which I thought needed to be exploited!

My original plan for the theme was a little more serious and philosophical: sometimes in life, a person (in this case the setter) has to tell white lies to someone they care about (the solver) in order to help them, for example tricking a drug addict into attending an intervention, and the person receiving the help needs to keep the faith that the lies are well-intentioned.

When Michael got on board with the idea, the theming became much more light-hearted as he blessed the puzzle with his inimitable finesse, and I'm very glad that it went in that direction - I think it's much more entertaining as a result!

You mentioned that setting this would require working backwards... to be honest, I got fairly lucky with the first draft, which I set "forwards", in that I always found places to put the twists I had planned and the draft was made quite quickly. Refining it, though, was one of the fiddliest exercises I think either of us have ever gone through!

We both worked extremely hard with the help of the testers to ensure that we crammed in as many twists as possible as elegantly as possible, whilst keeping it solvable and unique no matter how meticulous the solver is with their deletion of pencil marks and even digits (if they didn't clear fog) that retrospectively don't work with the updated rules. It was quite the challenge, but definitely worth the effort!

The only regret is that we weren't able to finish with Michael's hilarious idea of announcing that normal sudoku rules no longer apply for the last few cells! If there are any setters reading this who reckon they could get that to work with an unambiguous solve path, we need to talk!

Thanks again for your enormously entertaining solve of this puzzle, and more generally for providing such a wonderful platform for slightly mad puzzle addicts like myself to share our passion!

[jongurney]

That final reveal switching the lines when you recheck all the blue ones & they work is one of the greatest moments in Sudoku history

[MarcusTheDorkus]

Holy shit this puzzle was amazing. Every time I revealed fog I was shouting "you son of a bitch" and getting so giddy. What a clever construction.

[TheTastefirst]

Karl the Fog is the nickname for the nearly ever-present fog in San Francisco.

[sinerged]

This sudoku belongs in a museum. It cannot be categorized as anything else but a work of art. I'm not even exaggerating.

[bivsknf]

It's always fun to see how Simon becomes sad, when new digit doesn't clear the fog, when it couldn't 😁

[HunterJE]

Any time it's tempting to think "I'm not sure if I'm supposed to be able to see that" in a fog puzzle, I think it helps to consider all the very clever ways constructors have found since the development of fog sudoku to hide stuff they _don't_ want the solver to see yet, and give the constructor credit by assuming that if you _can_ see it they meant for you to see it

[davidalderson]

The reveal that the two colours of lines are the wrong way around is amazing.

[bluji]

This is easily one of the most entertaining sudokus ever solved on the channel. I cackled out loud several times with those fog reveals :D

[ronjohnson]

The look on Simon's face at the first trick thrown at him is priceless.

[thejuggler]

Thanks for doing, uh... "Karl's" puzzle! We're so glad you enjoyed. :)

(Regarding the emoji at the top: on our devices and in all the browsers we tried, SudokuPad renders it as a single "face in the fog" symbol: 😶‍🌫 Sorry for the graphics glitch!)

[martysears]

Amazing and innovative idea, BUT... in the hands of 99% of constructors, the execution would have led to a confusing and frustrating mess of a puzzle. To pull something like this off with such logical rigour and attention to detail is something that could really only have been done by two geniuses like Stu and Michael. For them to have kept track so intricately of what the solver would know at every step, which specific cells would be revealed, in what order, and how this affected and updated what is known. It boggles my mind to think about. Bloody love it, well done chaps. And Simon, your reactions to such nonsense, especially at those big plot twist moments, are always spot-on and incredibly satisfying to watch as the constructor of said nonsenses. That's why we love you

[tdbraun]

They lead the solver through the puzzle like a mystery author leads readers past all the red herrings and plot twists. Wonderful!

[hasui]

Love seeing Simon getting pranked by a Sudoku Puzzle :D

[PassionPopsicle]

"Please share this with your friends"

Simon, you assume I have friends who do variant Sudoku.

Indeed, Simon, you assume I have friends at all 😂

[sorellla]

I also watch at bed time, but has nothing to do (and I'm certain I talk for everyone else who does tha same) with you or Mark being boring 😂. It's because I need something calm and soothing but still entertaining to wind down from the day. It's like a relaxing night routine for me now and I'm so grateful for having found the channel ❤

[Glazer]

Completed the puzzle before watching the video, and now I've never been more excited to watch some complete a puzzle I know the solution to. What a treat

[MrWaffles]

Making art right is impressive enough. These constructors did something even more amazing, which is to do it wrong on purpose in a clever way. What an absolute delight.

[MarcusTheDorkus]

The thing at the top is supposed to be a face partially obscured by fog. But Simon's version of Windows doesn't support the emoji so it displays it decomposed as a face with no mouth and fog next to each other.

[keefyg]

I’ve not laughed so much doing a Sudoku ever. Flipping genius, well done setting that.