A polymer clay friend tweeted something about an optical illusion that is so neat I feel it has to be passed along.
Here's the link: The Blue and the Green
And here's the illusion:
Believe it or not, the colors you see as pale blue and lime green are actually the same color-- a slightly aqua shade of green. (Click the link above to see the exact color and a magnified portion of the original image.)
The trick is that (if you look closely, you'll see) the orange stripes go over the "green" spiral, while the fuchsia stripes cross over the "blue" spiral. The juxtaposition of the dark pink or orange against that single aqua-green color tricks our eyes into seeing them as two different shades. Amazing!
There's a link at the bottom of that page linked above, leading to this other interesting optical illusion: The Spinning Silhouette. It did take a little while for me to get the hang of it, so if you don't "get it" at first, don't give up!
Ok, just one more... Here's an "audio illusion" I found on YouTube: