The rules of the Moose game are as follows: you say the moose eats something, then something that puns or aptly describes what it eats. the format is: Moose eats X. Y, Y. for example: "Moose eats change. Irony, irony." It's that simple! There is no obligation to reply to anyone playing the Moose game at you, or to even accept that it is an interesting game.
See The Ravenous Moose for a story, and how the Moose game came about.