WHO. When the dog hears his master give the command "Who's that? Who's that?" while pointing to a particular person, it quickly springs to life and charges toward the poor chap.
MASTER. The master is, of course, just playing a game with his dog but the target always runs for his life and makes sure the dog doesn't get a piece of him.
FORGET. Moments later, the dog forgets about the whole thing, and the terrified person comes out of his hiding place.