The rules are simple. The first poster plays the role of the customer and requests a particular variety of cheese. The next poster plays the part of the shopkeeper who must offer an excuse as to why he does not have any of the requested cheese. The third poster will again play the customer requesting a different variety of cheese, and so forth and so on. For example:
Customer: May I have some Camembert.
Shopkeeper: I'm sorry, the cat has eaten all the Camembert.
The idea is to keep it going for as long as possible without repeating a variety of cheese or an excuse.