Can you fold a square sheet of paper into three equal rectangles, without using anything but folding?
(That is, you are not allowed to use anything but your hands, and you cannot do anything but folding... everything else is forbidden!)
Label the corners A, B, C, D clockwise starting in the upper left hand corner.
Fold and unfold C diagonally up to A along BD.
Fold and unfold CD up to BD and mark the point where this fold intersects BC, E.
Fold and unfold DE up to BD and mark the point where this fold intersects BC, F.
BF is 1/3 of BC
Perhaps easier to understand...
