The next aisle over, Peter touched my shoulder. He was holding a can of Purina dog food. ''Excuse me, Sara, but can you tell me what this is?'' Behind him, the pet food was stacked practically floor to ceiling. ''Um, that's food for our dogs,'' I answered, cringing at what that must sound like to a man who had spent the last eight years eating porridge. ''Ah, I see,'' Peter said, replacing the can on the shelf and appearing satisfied. He pushed his grocery cart a few more steps and then turned again to face me, looking quizzical. ''Tell me,'' he said, ''what is the work of dogs in this country?''