The question of a majority and certainty, and how it has impact on the assumptions that determine whether and how one needs to check for chametz. But what if you see a mouse bring chametz into one of two houses? What's the certainty? And what do you have to do? The Gemara notes that mice don't leave crumbs, as compared to a child who might have done the same thing - and leaving crumbs. Plus other mouse cases, when you can't be sure that the mouse and/or the chametz is the same that you saw going in. Also - a new mishnah: if you miss the correct time for checking for chametz, do it afterwards. It's still relevant!