Rumsfeld Hadrons

Donald Rumsfeld, in attempting to excuse the inexcusable, once (in)famously said that ``there are things that we know we know; there are things that we know we don’t know; and then there are things that we don’t know that we don’t know". Recent discoveries about hadrons with heavy flavours fall into those categories. It is of course the third category that is the most tantalising, but lessons from the first two may help resolve the third.