https://www.youtube.com/watch?v=TNvjAfKTeY0
The Pythagorean Theorem (a2 + b2 = c2) is the coolest theory I ever known so far. This small video proves that the Pythagorean Theorem is real. At the beginning, two squares of the equal area are shown. One of the squares split into two squares (look carefully!) You will realize that the area of the two smaller squares and the area of the biggest square can form a triangle in the middle.