 How and whether common knowledge can be achieved depends on available communication facilities. Public announcement or publicly observable ritual (the cashier ritual mentioned above) can create common knowledge. But, as Halpern and Moses (1984) proved, message exchange in a distributed environment, where there is no guarantee that messages get delivered, cannot. Halpern and Moses use the example of two generals who are planning a coordinated attack on a city. The generals are on two hills on opposite sides of the city, each with their own army, and they know that they can only succeed in capturing the city if their two armies attack at the same time. But the valley that separates the two hills is in enemy hands, and any messengers that are sent from one army base to the other run a severe risk to get captured. The generals have agreed on a joint attack, but still have to settle the time. So the generals start sending messages, for example, “Let’s attack at 9:00 AM”. But they cannot be sure that the messengers succeed in delivering their message. And if they get through, there is no guarantee that the message of acknowledgement will get delivered. And so on.