There are four soldier squads planning to attack a town from the north, south, east, and west at the same time.
They are in their positions; now they just need to know when to attack. One of the squads sends a courier went to tell the other squads when to attack. The problem is, he might die. So all the squads need to know exactly when to attack together so no squad will attack without the other. But then they need to know that all the other squads actually got this message, and the man didn't died, and so on.
How can they attack?