The status of floating impurity... the puzzles of the circumstances when it would extend impurity and when it would not. Plus, the implications for purification, as needed. Also, a nazir and a person busy preparing a Korban Pesach walk together by a "an impurity of the deep" (tehom)? Now what? Well, it depends on the point of their process of purification to begin with. Which leads to what might be the most polite, civilized (interactive) dispute we've seen in the Gemara.