I sometimes have trouble explaining things to Kid. Like why some movies are considered G and some PG. Why some books are "Early Reader" and others very similar are "9-11s." I can't quite think of why some of the Lego pieces are partitioned off in slightly differently textured bags with more ventilation.
I wouldn't be able to explain this to him either, but thankfully he is still in Grade 1 and not really into Renaissance books of manners yet.