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.