A spider and a fly sit in a closed room. What's the shortest crawl between them?
In a rectangular room 30 feet long, 12 feet wide and 12 feet high, a spider sits in the middle of one end wall, one foot from the ceiling. A fly sits in the middle of the opposite wall, one foot from the floor. The spider can crawl over walls, floor and ceiling but can't fly or drop. What's the shortest possible path to the fly?
Reveal the answer
The obvious route, straight across, is 42 feet. But unfolding the room's six faces flat and drawing a straight line between the two points reveals a shorter path across five faces: just 40 feet. Puzzle creator Henry Dudeney set this in 1903, and the two-foot saving still surprises people who trust their first instinct.
— Henry Dudeney, The Canterbury Puzzles — first published in the Weekly Dispatch, 1903