I used to work with a guy who stole my food all the time. Once I caught him dipping his fingers in my container of tuna pasta salad. "Oh," he said, "I thought it was mine." "Show me," I demanded. "Show me your identical container of tuna pasta salad." He couldn't and slunk away in shame. Not enough shame, however, as he continued to take my food until I told him one day I'd taken to spitting in it before bringing to work. (I wasn't really, but it seemed to do the trick.)