What is parallel ?

Two things are said to be parallel if they never intersect.  Two universes which are parallel would never overlap or intersect, because they are so different.  For those of you who are fans of The Big Bang Theory, here is an example of a parallel universe...

http://themetapicture.com/penny-and-sheldon-in-a-parallel-universe/
For those of you who haven't yet met Sheldon and Penny, here is what they normally look like.







Take a closer look at Sheldon's T-shirt.  It depicts some opti-blocks, impossible cubes reminiscent of Escher art. 
What is so impossible about the blocks?  Aren't they merely one block stacked on another?

To find out more about the math behind Escher art, check out this website:
 http://www.math.nus.edu.sg/aslaksen/gem-projects/maa/0203-2-03-Escher/main3.html