could be something...
stain [n.] a discoloured patch or dirty mark that is difficult to remove.
theory [n.] a reasoned set of ideas that is intended to explain why something happens or exists.
welcome to tea stain theory. you're probably here to find out more about it, or just what the hell it even is in the first place. we can help you with that. possibly.
So what are you both mad about?
SKANDAR: He loves tea.
SKANDAR: He's a big tea man.
You don't collect vintage teabags or teapots, do you?
WILLIAM: No, I wish I did.
-- from this interview