Two old ladies go visit their friend Mary. They sit down and after a while Mary says: "How foolish of me! I haven't even offered you coffee." So she gets up and gets the coffee.
Some time later, old Mary says: "How foolish of me! I haven't even offered you coffee." So again she gets the coffee. Half an hour later, the scene repeats for the third time.
Finally the two ladies say goodbye. "Mary is acting really weird, don't you think?" says the first one. "All the time we spent there, she hasn't even offered us coffee!".
The second one looks at her in amazement: "What? You've been to see Mary, and you haven't even invited me??"