The 12 Most Controversial Math Problems...

Upon seeing this headline, I clicked to see what I thought would be an article on problems that challenged mathematicians, but that's not at all what this article is about.  Instead, it's about math problems that challenge ordinary people's understanding – and it's quite well explained and illustrated.  About half of these problems are some I'd run across before as brain teasers; the other half were new to me.  All were interesting and illuminating.  Recommended reading!