here). It is one of many impossible objects that result from our visual system interpreting a possible 2-dimensional figure as a 3-dimensional impossible one. Last week I talked about Kanizsas triangle, an illusion in which our brains are tricked into perceiving contours where none exist. Christopher Tyler of the Smith-Kettlewell Institute has produced an image that combines the two.
To do so, he took a basic, though pretty cool looking version of Kanizsa's triangle (Tyler's original image with the white lines removed):
And added 3 white lines, to produce this (his original image):
The white lines produce a depth effect which makes the triangle appear 3D, and therefore impossible, as in the Penrose triangle. It took me a minute to get the effect, so look again if you don't see it the first time. I find it's best if you look just off center (e.g., at the center of the top left circle).
This illusion was one of the finalists in the 2011 Illusion of the Year contest (you can see a larger version of the image there).