Bongard problems

''A Bongard problem is a kind of puzzle
...
The idea of a Bongard problem is to present two sets of relatively simple diagrams, say A and B. All the diagrams from set A have a common factor or attribute, which is lacking in all the diagrams of set B. The problem is to find, or to formulate, convincingly, the common factor.'' [source]

Test yourself with these amazing problems:
http://www.foundalis.com/res/bps/bpidx.htm
(some are less evident than others!)

Example:

My personal solution is: the objects on the right have 3 sides, the ones on the left all have more than 3 sides.


Check this article from Hofstadter.



There is even a game based on these problems: Zendo.