By Daniel Currie Hall. Suppose time stops and the white knight can make as many consecutive moves as it pleases. How quickly can it mate the black king provided that it never moves onto a square on which it’s under attack? (It can make captures, provided it makes them “safely.”)
King William’s College, on the Isle of Man, has posted this year’s edition of “The World’s Most Difficult Quiz,” with its customary epigraph, Scire ubi aliquid invenire possis ea demum maxima pars eruditionis est (“The greatest part of knowledge is knowing where to find something”). Some sample questions:
Answers will be posted at the end of January.
Usually MetaFilter organizes a Google spreadsheet of communal guesses; if that materializes I’ll post a link here.
A reader nicknamed MANX submitted this poser to The Enigma, the magazine of the National Puzzlers’ League, in September 1985.
The letters in BENEATH CHOPIN can be rearranged into a fitting three-word phrase of 3, 5, and 5 letters. What is it?
Reader Chris Dawson has devised a tiling puzzle game with a twist: Players drag, rotate, and scale pieces to fill a grid, but each piece can be scaled to either 1x or 2x its base size.
“The scaling mechanic doesn’t just add variety — it fundamentally changes the maths of the puzzle space. Scaling creates a solution space that grows faster than puzzle complexity itself. In a minimal 4-piece puzzle [below], adding scaling provides a modest 3x multiplier. But add just 2 more pieces, and that multiplier explodes to 21x — a 7-fold amplification. This isn’t additive enhancement; it’s exponential transformation.”
Here’s a demo, and here’s the beta, with daily challenges. A multiplayer version is in development.
(Thanks, Chris.)
This year’s GCHQ Christmas Challenge is now live. Devised by Government Communications Headquarters, the British intelligence agency, this year’s quiz presents seven puzzles for children aged 11-18. They’re designed to test a range of problem-solving skills, including creativity and intuitive reasoning.
Agency director Anne Keast-Butler said: “Puzzles are at the heart of GCHQ’s work to keep the country safe from hostile states, terrorists and criminals; challenging our teams to think creatively and analytically every day.”
Draw a circle and choose 100,000 points at random in its interior. Is it always possible to draw a line through the circle such that 50,000 points lie on each side of it?
In Arthur Ransome’s 1933 children’s novel Winter Holiday, Nancy Blackett, quarantined with mumps, sends a picture to her friends of a sledge being drawn by skating figures. Nancy is encouraging the group to pursue their plan to explore a frozen lake. The seven figures in the picture correspond to the seven children in the group. “But,” asks Peggy, “what did she put in the crowd for?”
Mathematician Matthew Scroggs’ 2025 advent calendar offers 24 puzzles, each of which has a three-digit answer. The answers will help you order the parts that Santa needs to rebuild his sleigh and save Christmas.
A new puzzle is revealed each day. Prizes will be awarded to 10 entrants who successfully build a sleigh before the end of the year. Details are here.
In 2012 I mentioned that Helen Fouché Gaines’ 1956 textbook Cryptanalysis: A Study of Ciphers and Their Solution ends with a cipher that’s never been solved. Reader Michel Esteban writes:
I think I found what kind of cipher Helen Fouché Gaines’ last challenge is.
In my opinion, it is a seriated Playfair of period 5 with two peculiarities:
– Zs are nulls in the ciphertext,
– Z is the omitted letter in the cipher square (instead of J).
If I am right, period 5 is the most likely reasonable period: we can observe no coincidences between upper and lower letters.
On the other hand, six reciprocal digrams appear: FD-DF, EC-CE, JN-NJ, JB-BJ, QL-LQ and GW-WG. These are almost certainly cipher counterparts of common reciprocal digrams (ES-SE, EN-NE, IT-TI, etc.).
I did not solve this cipher, because it is too short to use statistics. The only way to solve it is to use some metaheuristics (like Hill Climbing), but I have no computer!
I have no doubt you know someone that will be able to unveil the plaintext after having read these considerations.
Can someone help? I’ll add any updates here.
A wazir is a fanciful chess piece that can move one square horizontally or vertically, but not diagonally. This one finds itself in the upper left corner of the board. Can it make its way to the lower right while visiting each square exactly once?
