The Penrose Triangle (aka "Fun With Dimensions")

You know what impossible figures are, right?

No, not Barbie dolls, I mean, in the cognitive or math sense, as in - something that is physically unachievable, but because you're looking at it in 2-D on the screen, seems reasonable.

Ok, fine, Barbie counts too (as do comic characters, the female ones in particular.  Also, anything by Rob Liefeld)

The one to the right has always been one of my favorites - its a Penrose Triangle.
Yup, that Penrose.  The one with the seriously awesome mind and the Kleenex lawsuit.

It is seriously cool, and has even made it onto a postage stamp :-)

 

Incidentally, if the above looks vaguely Escher-like, its not surprising - it was inspired by some of Escher's works.  

Incidentally, Escher's most famous paintings (the staircase one, and the waterfall) were inspired by an article that Penrose wrote.  From Teh Wikipedia

As a student in 1954, Penrose was attending a conference in Amsterdam when by chance he came across an exhibition of Escher's work. Soon he was trying to conjure up impossible figures of his own and discovered the tri-bar– a triangle that looks like a real, solid three-dimensional object, but isn't. Together with his father, a physicist and mathematician, Penrose went on to design a staircase that simultaneously loops up and down. An article followed and a copy was sent to Escher. Completing a cyclical flow of creativity, the Dutch master of geometrical illusions was inspired to produce his two masterpieces.