The Colour Therapy Professionals & Colour information resource This is an amazing illusion using colour and tone to deceive the eye.
Take a look at either of the cubes below.
Study the middle squares of each face of the cube, and decide if they are the same colour or tone depending on which cube you look at.
Once you have done this, then move your mouse over one of the middle squares of the cube and drag it to another middle square.
The reason:
The surrounding colours of different tones, contrast the lighter middle squares. This is less noticeable to the eye when the lighter square is surrounded by similar tones.