Euclid's proof of Pythagoras' theorem

This is an illustration of the classical proof of Pythagoras' theorem. There are many other proofs, but this is the one from Euclid.
Play around with the vertices, A, B & C. Can you describe carefully the construction shown here? Why are the four coloured triangles equal in area (green=yellow=pink=blue)? Can you find a rectangle having the same area as the square BCIH? How can you conclude that ?