Which of these blocks is darker, the top or the bottom? It seems obvious that the top block is clearly darker than the bottom, but what if I told you your eyes aren't telling you the truth. The two blocks are, in fact, the exact same colour and only appear different due to the effect of the darker and lighter shading across the middle.
We perceive the upper square to be darker and the lower one to be lighter because that's what our brains expect due to the other elements surrounding the grey. Our brains think it's lit from above - but it's not.
Don't believe me? Place your finger across the centre of the blocks totally covering where the blocks join together, and the real colours of the grey blocks will be revealed.