The phrase "think outside the box" is very overused... and yet we humans do tend to get stuck in mental ruts. Just like ruts for wagons, mental ruts are often a useful guide that get us where we want to go without much effort. But sometimes it's nice to go off-roading, so here are some "lateral thinking" puzzles to ponder.
Mental ruts are especially interesting from an artificial intelligence perspective. Getting a machine to "think inside the box" and come up with "obvious" answers is quite hard. Most AI systems that are in any way "creative" spend so much time so far out of the box that they aren't especially useful.