The brainteaser tag looks like it's heading for being our catch-all tag here.
I don't really understand what “brainteaser” means. It's not that far from being a synonym of “puzzle”.
A brain teaser is a form of puzzle that requires thought to solve. [Wikipedia]
a problem for which it is hard to find the answer, especially one which people enjoy trying to solve as a game [Cambridge)
Should we have this tag at all? If so, what should it mean — what types of puzzles do and do not qualify, and when should questions get that tag? Once we get a consensus, this needs to be clearly explained in the tag wiki.