An interesting thing about idioms is that even though they are made up of more than one linguistic unit (the morphemes that build the words that build the phrase), they are generally treated as one unit. If a word or even a morpheme that grammatically marks a word in an idiom is changed, it usually loses its idiomatic meaning. For example: when wishing an actor good luck we say, "break a leg," but if the actor did well we would not say, "he broke a leg." Also, you can "beat a dead horse," but not "beat a dead cow," or "beat two dead horses." And, it is the "dog days of summer," not the "cat days of summer," or the "dog month of summer," or even "a dog day of summer."