The chessboard is made of alternating light and dark squares, with square number 1 belonging to the dark set and square number 2 belonging to the light set. However, if you drag the image reveal slider you’ll see the two squares joined together – they’re the SAME COLOUR!
Based on an illusion published in 1995 by Edward H. Adelson, Professor of Vision Science at MIT, it’s the shadow from the head that darkens the central squares and balances the colours on squares 1 and 2. However, our brain makes assumptions about the nature of the chessboard and tells us that they MUST be very different shades.

