Optical illusions are images designed to trick your brain into seeing something that isn’t really there. Because of the way your brain works, even when you know the trick, it’s almost impossible to train your eyes not to see the illusion. Trying to force your brain to cope with two conflicting ideas at the same time is hard work. Look at the grey lines. Are they parallel or not? Find a ruler with a flat edge and you can check that these lines really are parallel, even though they appear slanted.