Four spies share secrets over the phone: each call updates both participants on everything the other knows. The question is how many calls are enough for no one to ignore anything.
The relay of messages
Riddle statement
Four spies—A, B, C, and D—each have a different secret.
Every time two spies talk on the phone, they both tell each other everything they know at that moment.
What is the minimum number of calls necessary for all four to know the four secrets?