tl;dr:"Imagine there’s a city in a valley. On either side of the valley, there’s an army commanded by a general. On one side stands General Alice and her army. On the other, General Bob and his army. Alice and Bob want to capture the city, but neither has an army large enough to do so alone. Alice and Bob must attack at the same time, but Alice and Bob can only communicate by sending messengers through the valley, who have a chance of being captured by the city’s army. How do Alice and Bob coordinate their attack?"