When foods get mingled, and you need to determine whether you can eat the kosher food, or whether the contamination of the non-kosher with the kosher. Plus, how to pull away the non-kosher/forbidden fat, and leave the kosher meat behind, with no flavor of the forbidden fat added to the taste of the otherwise kosher food. Determining whether the non-kosher food imparts flavor — one needs a non-Jewish “taster,” a gourmand or a chef, who knows the taste of the non-kosher food and has the ability to determine whether that non-kosher taste is present in the kosher food (and therefore rendering it non-kosher). Also, determining how much non-kosher food can be present in a mixture with kosher food, primarily via whether it imparts flavor. Which leads to the discussion of 1/60th of the sum total. And the classic example of a cow’s udder, that is meat and has milk in it.