vanishment puzzle

A vanishment puzzle is a mechanical puzzle in which the total area of a collection of pieces, or the number of items in a picture, appear to change following some manipulation. A well-known puzzle, first seen in 1868, involves an 8 × 8 square that is divided up into two triangles and two trapezoids. The pieces are reassembled into an oblong shape that measures 5 × 13. Where did the extra bit of area come from? The answer is that final shape is not perfectly rectangular but instead has a narrow gap that runs the length of one of the diagonals. Most famous of all vanishment puzzles is Get Off the Earth.