This illusion has been doing the rounds this week (see Bad Astronomy and Richard Wiseman for a couple of science blogs I like that picked it up), but it's so good I thought it needed to be posted here also.
Look carefully at the image below. Do you see a couple of spirals, one blue and one green? Well, take a closer look - in actual fact, the blue and green are actually the same colour!
Don't believe me? Copy the image and open it up in PhotoShop or Paint and take a closer look....
You will notice that the orange curves move through the "green" spirals, but not the blue. And the purple curves don't move through the green.
If we blow this picture up even more, we can see that the colours are becoming more and more similar.
The blue and green appear to be different colours because our brain works out colours by comparing them to other surrounding colours and it does a bit of mixing. When we look at the "blue" spiral, we also take in the purple curves moving through it. This makes it look more blue. When we look at the "green" spiral, we take in the orange curves, which makes it look more green.
I know that's not a great explanation, so I'd be happy to hear a better one!
Edit by westius 2/7/09: If you doubt this illusion, check out this image - I've replaced some of the colours - you can clearly see now that the 'blue' and 'green' are the same: