בבא קמא ע''א
תוספות ד''ה אצטריך
It occurred to me today that תוספות and the last part of the מהגורא בתרא של המהרש''א are dealing with two different questions.
The question of תוספות is what I want to present right now. It is simply this take the חלק שלישי of the גמרא and insert it into חלק שני and you get a question. The חלק השלישי said you get שן and רגל from one פסוק. The second part said you get foot from one and tooth from another פסוק. Now take that one פסוק that we are using for both and ask the same question that the Gemara asked before.
That is now in חלק השלישי we use ושילח for both. Now that is good as far as רגל goes. But what about שן? We used וביער for שן. And we showed it had to be used for שן because of  כאשר יבער. And without that פסוק we would have been justified in using it for רגל. That is an open גמרא. But now insert ושילח instead of וביער. What can you answer now? We don't have anything to prove that ושילח goes on שן.

(1) חלק שלישי של הגמרא is like this.

שאלה: Write ושילח alone and use it for both, for we have two verses showing it can be used for both.

תירוץ: We would think it applies to one alone. The גמרא shows some reason why we would use it for tooth because it has pleasure and so it finds some reason for foot also.

שאלה: We if we would have one we would have to include the other because they are equal in severity.

תירוץ: If we would use it for both we still would not know that the owner is liable when it walked by itself.

(2) חלק שני של הגמרא of the גמרא is like this. אמר מר. We have וביער for שן, and we have proof: כאשר יבער.
שאלה: Why do you need proof? What else could you use it for?

תירוץ: We could use it for רגל.

שאלה: But we already have foot.

תירוץ: We would divide foot into two parts. (We would say the verse for foot ושילח is only for when he sent his animal to graze in someone else's field. We would not know if it walked by itself there and ate. And we would use וביער to tell us that last case.)

שאלה: Now that you have shown you use it for tooth now how do you know that when the animal walked by itself that the owner is liable?
Answer: היקש we make a comparison between foot and tooth. Just like tooth is liable whether he sent it or not so foot is liable in both cases.