It's commonly used as a brain teaser to test pupils' logic skills at school, and children seem to be able to solve it in an instant.
But despite it being a breeze for youngsters, a puzzle which asks you to identify which direction a bus is travelling in often leaves adults scratching their heads.
National Geographic has created a version of the popular puzzle showing a yellow bus with identical windows at either end - giving no clue as to which is the front or back of the bus.
The correct answer is that - if you're in the UK - the bus is travelling to the right. And the key to solving the puzzle is the fact that you can't see the passenger doors.