In year 12 my maths teacher gave the class this puzzle. The idea is to fold an A4 sheet of paper, along the lines, such that the numbers are in order. Just to make it clear, the numbers don't have to be in order as you unfold it or anything, but when folded the rectangle of paper directly underneath "1" has "2" on it, and directly underneath "2" has "3" on it etc. He hadn't been able to solve it himself and none of his classes could solve it. It took me about 20 mins to get it but I think some people may get it straight away. It came from a problem book for grade 2 students.