What levels of tumah for which kinds of foods (consecrated, terumah, chullin)? And what happens to kadosh (consecrated) items that come in content with impurity? The combination and contamination within one vessel. Plus the non-food impurity rendered by a t'vol yom. Also - the scenario of a needle in a cow, which is metal... What is pure, what is impure? Doesn't it matter where the needle is found? What's What: "Tumat Yadayim." One of the many rabbinic decrees that establish Impurity. Which brings us back to R. Hanina S'gan ha-Kohanim. Also, the question of "safek" - when there's a question about the status of something that is found, and the determining factors relate to environmental, surrounding norms. For example, utensils found on the path to or from the mikveh.