"Well, work it out. If Albert married Elaine, then Bert married Francine, and vice versa."
I said, "That's not enough; can you give me other hint?"
"Right. Either Bert married Daphne and Charles married Elaine, or neither of these couples happened."
"Still not enough, give me something else."
"Let's see... Charles married Francine if and only if Albert married Daphne."
"You are still not helping. Couldn't you tell me at least one specific couple, any one, so I can work out the other two myself?"
He smiled. "That wouldn't help you."
I thought a little, and I knew the three pairs. Who married whom?