Pesachim 46 - Deaf dough
If a batch of dough became "deaf," that is, if there are reasons to suspect that it leavened (such as if it became white, but has not shown surface cracks typical of leavening), then we take another dough that was kneaded in a similar situation and check that one for chametz. If the other dough leavened, we assume that the one in question leavened also. But what if we don’t have another dough? Then we assume that the dough leavens in the time it takes one to walk a "mil", or about two thousand steps, which is eighteen minutes. The same eighteen minutes appear in many other laws.
If one bakes dough on Passover itself, and this dough is ritually impure, as all our doughs today are, then he has a problem with "challah", the kohen’s portion, as follows. He cannot bake it, because one can only cook foods that will be eaten, and impure challah needs to be destroyed. Nor can he leave it over, because it will leaven. Rabbi Eliezer suggests to not declare the challah portion until all is baked, and only then combine all the matzot in the basket and separate one as challah (by now it is not chametz), and destroy it. Ben Beteira suggests putting it in cool water place until the end of the Yom Tov, when one will be allows to bake it. Finally, Rabbi Yehoshua says that this challah can be separated, and if it leavens - let it. Why? It does not belong to the owner, because it is for the kohen. Nor does it belong to any specific kohen. Therefore, this is not chametz that was prohibited by the Torah.
The Talmud discusses why each of the disputants does not accept the solution of the other two .
Art: The Bakers Cart by Jean Michelin