Originally posted by wittywonka
This may have been posted earlier, but I have not seen it, so here it is anyway. Take a look at the site below.
Got an explanation?
Yes. The slopes of the hypotenuses of the two inner triangles are not the same. Thus, neither combination of the 4 shapes makes a perfect right triangle. The top combination is like a right triangle with the hypotenuse bowed in, and the bottom combination is like a right triangle with the hypotenuse bowed out. This subtle difference in area compensates for the 'missing square' in the bottom combination.