A castle window divided into eight equal triangular panes
Sir Hugh de Fortibus shows his builder a one-foot square window, barred into four equal lights, each half a foot per side. Pleased, he orders a second window, set higher in the wall: its four sides must also each measure exactly one foot, but the bars must divide it into eight lights, every side of every light the same length. The builder protests it can't be done. Can it?
Reveal the answer
Yes — because Sir Hugh never said the second window was square, only that its four sides were each a foot long. Make it a rhombus (a "diamond" with 60°/120° angles) instead: it splits along its long diagonal into two equilateral triangles, and quartering each the usual way gives exactly eight small equilateral triangles, six inches to a side, matching Dudeney's own figures. From Henry Dudeney's The Canterbury Puzzles (1907), puzzle No. 36, "The Donjon Keep Window."
— Henry Dudeney, The Canterbury Puzzles — Puzzle No. 36, "The Donjon Keep Window," 1907