Pythagorus proof

What you see is:

1. A right angled triangle
2. Now a square is constructed on its hypotenuse
3. Then the triangle is flipped inside the square
4. Then three more identical triangles are constructed inside the square. The short sides line up with the middling length sides because the sum of the internal angles of a triangle add up to 180°
5. Then the top triangle is swivelled around to the left side