The Guardian – Alex Bellos
“For today’s puzzle, let me introduce you to the Menger sponge, a fascinating object first described by the Austrian mathematician Karl Menger in 1926. We’ll get to the problem as soon as I explain what the object is. The Menger sponge is a cube with smaller cubes extracted from it, and is constructed as follows: Step A: Take a cube. Step B: Divide it into 27 smaller ‘subcubes’, so it looks just like a Rubik’s cube.”(more)