There is a 'missing' square. It's hiding in between the shapes in the first one, but rearrange them to be tight, and it becomes clearly visible. It's called the "Missing Square Puzzle" (nice name). Basically, the two triangles are not similar triangles (in a mathematical sense), meaning that their sides are not proportional to the other triangle's sides. This means that the hypotenuse (long side) of the first diagram is bent slightly inwards, and the hypotenuse of the second is bent slightly outwards. The difference of the two angles makes almost exactly 1 square. http://en.wikipedia.org/wiki/Missing_square_puzzle