Happy birthday, Martin Gardner (1914-2010) !

Today is Hexaflexagon Day - around the world math enthusiasts are celebrating birthday of Martin Gardner. You can see and listen Martin Gardner on Nature of Things. or read an interview with Martin Gardner. Of course, I cannot believe if you are reading this blog and have not joined those many millions on YouTube who has become fans of hexaflexagons watching Vi Hart's videos on hexaflexagons.
My first introduction into "recreational mathematics" was in 1966 when I as a sixth grader won my first math olympiad. My teacher gave me a little book by Martin Gardner. It was translation in Latvian of Martin Gardner's 1956 book:

There are still  only two books by Martin Gardner that are translated in Latvian (the other one is Relativity Theory for everyone). The other books by Martin Gardner on my bookshelf in Riga were all in Russian. All those books are partly responsible for me becoming a mathematician (like for many others). Unfortunately university mathematics was nothing like the exiting topics from Gardner's books, so I was discouraged to become a research mathematician. Instead I decided to become a teacher so that I can teach mathematics in fun and accessible ways. My teaching carrier in school was not too long - about 5 years as part time job. Most of my teaching has happened in universities  - University of Latvia and Cornell University. And I did end up with PhD in Mathematics but that is another story.
Martin Gardner is certainly responsible that I always tried to make math visual and tangible.
So my tribute for Martin Gardner's birthday is this online exhibit.

This is how many people see mathematics - "I have no idea what is about".
The following paintings are all having something in common. Can you figure out what it is?

 

If so far you have not figure it out - here is a clue: