When you fill in a crossword, it can suddenly go from horribly difficult to extremely easy. This is similar to how some physical systems undergo phase transitions, but compared to them these puzzles can be mathematically unique.
One day when I was solving a crossword, alexander hartman At the University of Oldenburg in Germany, he came up with a personal challenge. I wanted to find enough words to create large, connected islands of letters. Then he realized that he had inadvertently run into a kind of well-known “percolation problem.”
Source: www.newscientist.com