Thank you so much for this. Is it fair to say, that on Page 30 of Sotah, they discuss dough, and not flour, because dough is wet, and therefore can be a rishon.
I am not clear about flour -- if it is dry, are you suggesting that it cannot be a rishon, but only a sheni. And if a dead frog falls in it, does one have to throw out all the flour, or just the frog. Thirdly, if a dead frog fall into the dough, as distinct from the flour, does one through out the dough, or just the frog?
I appreciate the time you gave/give to answer my questions. I am new to gemarah studies, so please excuse any questions that seem stupid.
I think you are correct that the case in Sotah is only possible because the dough was wetted.
A frog is not a Sheretz. A Sheretz which causes Tumah is only one of the eight Sheratzim mentioned in the Torah in Vayikra (ch. 11). However, let us replace a frog with a mouse. Neither the flour nor the mouse would have to be thrown out, as the flour was never wetted and is not Mekabel Tumah at all through a Sheretz, and there is no reason to throw out the mouse (besides to make sure he doesn't make anything else Tamei). If a dead mouse would fall on to dough, he would indeed make the dough a Rishon.
All the best,
P.S. Good Questions!