Quote - I'm surprized noone brought up the painting cats yet!
http://www.monpa.com/wcp/

 

It wasn't until about 2/3 of the way through the book that my wife and I figured out which of the following was true:
a.) This was a hoax
b.) Cats actually paint
c.) The authors are deluded
d.) It was a joke.

It was only a footnote which enabled us to decide exactly which of the above was true.

The sequal (Why Cat's Dance) was more obviously intended as humor, but it was still unclear whether or not the humans who dance with the casts were real (albeit mostly nutjobs) or if the whole thing was a put-on.