These two blocks are actually the SAME shade of grey. (Just slip your finger over the join)
It's hard to comprehend, but even though our eyes tell us that one block is darker and the other is lighter, we shouldn't believe them.
The two blocks are, in fact, the exact same color and only appear diferent due to the effect of the darker and lighter shading across the middle.
Place one finger along the center, blocking the darker and lighter parts, and your eyes will begin to tell the truth.