Subtleties of language

On the way home from school the other day we had our windows rolled down when we pulled up to a red light. A very loud Harley (is that redundant) went through the intersection and Brannon let out a "Holy crab". (Did I mention that he's three?) I turn back to look at him and he looked so proud that he had a new phrase. I had to turn back around to compose myself because I was about to bust out laughing. I then went into a five minute discussion about why we can say crab but not holy crab and we can say holy but not holy with anything after it and why we can say crab but not crap. In other words I don't think he understood any of what I was saying. Now he goes around saying crabby, crabby, crabby and holy, holy, holy while looking slyly at me to see if he is tip-toeing around the rules enough not to get in trouble.