This video has been posted at lots of blogs (1.8 million views) this past week, but I want to store it here as well. An informed commentor at Reddit explains that the toothy grin made by Denver...
"is known as "grimacing" and in the breeds that demonstrate it, it is a sign of submission."So the dog may be responding to the filmmaker's tone of voice rather than actually expressing guilt. Maybe Macy is the actual thief.
Addendum: A big tip of the hat to Daniel, who found a report of a study which may support my speculation -
Dogs looked most “guilty” if they were admonished by their owners for eating the treat. In fact, dogs that had been obedient and had not eaten the treat, but were scolded by their (misinformed) owners, looked more “guilty” than those that had, in fact, eaten the treat. Thus the dog’s guilty look is a response to the owner’s behavior, and not necessarily indicative of any appreciation of its own misdeeds.