98:11 One may separate challah from dough that was kneaded on yom tov. One may not, however, burn the challah on yom tov because holy offerings (kodshim) are not burned on yom tov. One may likewise not bake the challah portion because it's not considered fit to eat since we are all impure from corpse contamination. The challah portion may not even be handled on yom tov, so while it's still in his hand, one places it in a safe place until yom tov is over, at which point he burns it. (Even after he declares a portion to be challah and it’s still in his hand, he may carry it to wherever he wishes to deposit it – Mishnah Brurah 506:29.) If dough was kneaded before yom tov, one may not separate challah from it on yom tov. Rather, one bakes his dough and eats what he baked but he puts a little of that bread aside. After yom tov, he separates challah from that. One must put aside enough bread that he can separate challah from it and still have some left over. (This course of action is for outside of Israel, where taking challah is required rabbinically. For how to act in Israel, where this mitzvah applies Biblically, see MB 506:21.)
98:12 It is prohibited to knead clay on yom tov, even through a non-Jew. Therefore, if one needs to seal the oven in which he will keep food warm for Shabbos (or for yom tov itself – MB 507:36), he must prepare the kneaded clay before yom tov. (Editor's note: obviously, this is not the way our modern ovens work.) Taking mud from the street for this purpose is likewise prohibited unless he prepared it before yom tov and put it aside. One must be careful not to spread the clay or mud; rather, he must seal the openings without spreading it because smearing is a prohibited form of labor even when it comes to food preparation.