This is an amazing illusion using colour and tone to deceive the eye.
Take a CLOSE look at the cube below. Study the middle squares of each face of the cube, and decide if they are the same colour or tone as the other middle sqaures.
Once you have done this, move your mouse over one of the middle squares of the cube, and using you mouse, left click on it and whilst holding, drag it to another middle square of what you think is a similar colour. You should then see that both the middle squares are the same! - How strange is that!