Logically Addressed Question
Truthful Tina and Lying Leah are identical twins whose parents have separated; Tina lives with her mother and Leah lives with her father.
Susan the school secretary, who is known to be particularly clever and a master of logic, wishes to send letters to each of their parents. Although she has two addresses in the school file she doesn't know which address belongs to which parent.
Susan finds one of the girls in the playground but has no idea if she is talking to Tina or Leah. She shows the girl one of the addresses and asks her a question. She thanks the girl and returns to the office able to correctly address both envelopes.
What question could Susan have asked?
Susan shows the girl the address of one of the parents and asks the question: "Is this your address?"
It it were the father's address then both girls would say, no.
If it were the mother's address then both girls would say, yes.
So from the response Susan is able to logically deduce which address belongs to which parent. However, she has no idea which girl she spoke to.