A stunning perceptual sleight of mind from John Sadowski. It's been doing the rounds on the web but is seriously worth your 30 seconds:  <a href="http://www.johnsadowski.com/big_spanish_castle.html" target="new">Spanish Castle</a>

And if you liked that, here's buckets <a href="http://illusionsetc.blogspot.com/2004/01/site-map.html" target="new">more</a>!

Thanks <a href="http://www.badscience.net/" target="new">Ben Goldacre</a>