Between pure & impure

The last pasuk of Shemini says the whole parsha was given to distinguish between the pure and the impure. But Rashi asks: is that really needed to tell a donkey from a cow? He reads it as pointing to something much finer, a difference as small as a hair's breadth. The Chofetz Chaim's preface to the Mishnah Berurah applies it to Shabbos and speech. A man opens his door to a rosh yeshiva's son collecting for the yeshiva, and asks what the second seder is worth to his father. The reply makes that hair's breadth very real.