It is a parlour game. But everyone loves to be tricked. Artists have been playing these kind of tricks since perspective was brought into painting. The thing is you trick yourself. I think everyone becomes a child when they look at these illusions. Its pure delight.
A little video about an illusion. Art? or an ornament. http://newopticalillusions.blogspot.com/2006/10/hollow-face-illusion.html
The ball and cup illusion http://newopticalillusions.blogspot.com/2010/08/cup-and-ball-trick-optical-illusion.html
the dragon’s head. http://newopticalillusions.blogspot.com/2006/11/dragon-illusion.html