Very cool illusion

A friend sent me a link to one of those optical illusion thingies. I'm often very sceptical of them however upon inspection, this one seemed different and legitimate. I don't think its fake - but I equally don't understand why its screws with your eye's so much! Here are the instructions from the website…

If your eyes follow the movement of the rotating pink dot, you will only see one colour, pink. If you stare at the black + in the centre, the moving dot turns to green. Now, concentrate on the black + in the centre of the picture. After a short period of time, all the pink dots will slowly disappear, and you will only see a green dot rotating if you're lucky! It's amazing how our brain works. There really is no green dot, and the pink ones really don't disappear. This should be proof enough, we don't always see what we think we see.

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Optical Illusion