Euclid's proof the the pythagorean theorem T-Shirt

Although Euclid's geometric proof of the pythagorean theorem is not the simplest or most intuitive, but it is the end product of a series of "propositions", each building on the previous ones. In this case, the square on the hypotenuse is divided into two parts, each corresponding to the area of the squares on the other two sides. It takes quite a bit of mental juggling to see that the two oblique triangles are congruent and that the pink areas are equal in area because of this congruence. It is fun to let someone figure it out and suddenly "see" it.