We learn about cases where a fermenting agent falls into dough, and where wine or vinegar falls onto lentils or grits. The rabbis consider the temperature of the food (heated food is more susceptible to blemish), the order of things that have fallen into the food, and whether two incidents might join together.
The entirety of this discussion assumes that the reader understands rabbinical thinking regarding whether a sharp taste might somehow taint a food. Further, the Gemara considers which substance fell into another, and which substance received the substance. Each of these are affected by the rabbis' guidelines.
Rabanan teaches that beer was forbidden when a mouse fell into that beer. Does this mouse improve the taste of the beer? Is it considered to be repulsive, though some kings serve mice (or perhaps squirrels)? Is it metamei, ritually impure? The rabbis remind us that some things are metamei only when they are moist. When they are dry, they are permitted. This leads the rabbis to discuss seminal emissions. Isn't semen only ritually impure when it can be used to conceive; when it is moist?
The rabbis attempt to understand what their own Sages had thought regarding substances that were permitted, forbidden, and ritually impure.