Look at the grey ring below. Are the two sides the same colour, or is one side darker than the other? Move the horse's tail out of the way and see. Try moving just the grey ring.
When the horse's tail is there, most people think one half of the ring looks darker than the other half. When you take the tail away, most people see the ring as one shade of grey.
If you would like to know more about why this happens, read the explanation.