In times without Internet, telephone or even reliable mailing system, this was an ingenious way to test if an unknown visitor could speak their tea language and was one of them or not.
In these 2 pictures, for example, I have set up the cups to test you. Imagine that the boss has just filled them and asks you to pick one. Which one will it be, if you want to tell him, without words, that you're one of them? (One or more answers are possible for each picture).