When dough remains in the grooves of a kneading trough, if there is as much as an olive's [volume] in one place, he must remove [it]; but if not, it is null because of its insignificance. And likewise regarding the [laws of] contamination. If he objects to it, it interposes; but if he desires it to remain, then it is as the trough.  [Regarding] “deaf” dough, if there is [dough] similar to it that has already leavened, it is forbidden.

How do we separate challah from contaminated dough on the festival?  R' Eliezer says: She should not designate it with the name [challah] until it is baked. R' Yehudah ben Beseira says: Let it be cast into cold water.  Said R' Yehoshua: This is not the leaven concerning which we are warned, It shall not be seen (Exodus 13:7) and It shall not be found (Exodus 12:19). But he separates it and leaves it until the evening; and if it leavens, it leavens.