What are some problems that can easily be carried around in your head and require no need of ink and paper? Problems like irrationality of $\sqrt2$ or infinitude of primes or Gauss's first initiation with number theory.
I had and lost Mathematical Snippets. So I was hoping for something similar reference. Simple problems that I can take to shower, or during rush hour traffic, or for a walk, or waiting in a line in grocery or during a math lecture to entertain me. /nodisrespecttoprof. :]
Thank you so much in advance!
EDIT: I am sorry, I actually had Math-A-Day in mind. But being a fan of Theoni Pappas I also bought the other one I mentioned.