Grammar Guru

"Why does that sign say, 'Beware of Dog'?" Zach asked as we pulled into our driveway today and he pointed to our fence. Aaron began to explain what beware means but Zach interrupted him and said, "I know what beware means," in an exasperated tone. He continued on saying it doesn't make sense to say Beware of Dog but that instead it should be Beware of the Dog, unless of course your dog's name is Dog, then it could say Beware of Dog. Does anyone else have a five year old who corrects grammar?