Pesachim Chapter 2, Mishnah 8. This mishnah deals with two unrelated topics: the concern that certain mixtures will become chametz, and the prohibition against cooking the Pesach offering.
The concern of chametz in charoses and in mustard:
The mishnah opens: "Ein nosnin kemach l'soch ha'charoses o l'soch ha'chardal" - one may not put flour into the charoses or into the mustard. The charoses referred to here is similar to the charoses we eat on the night of Pesach: any kind of crushed fruit mixed with a little vinegar and water. One may not put flour into it, and likewise one may not put flour into mustard. And if he did put it in - "yochal miyad" - he must eat it immediately, since the mixture is liable to become leavened.
The Gemara explains that the permission to eat it immediately does not apply equally in both cases:
With mustard: here the ruling that he may eat it immediately does apply, and eating it right away is sufficient.
With charoses: there is a concern that it will become leavened even immediately, faster than water, whose measure is eighteen minutes, and therefore it may not be permitted even to be eaten right away.
Rabbi Meir disagrees and holds that with mustard too, when flour was put into it, it may not be eaten immediately, just like the law of charoses.
Cooking the Pesach offering:
From here the mishnah moves on to the laws of roasting the offering, which must be roasted and not cooked: "Ein mevashlin es ha'Pesach lo b'mashkin v'lo b'mei feiros" - any liquid in which one cooks is considered cooking, and the matter does not depend specifically on water.
On the other hand, the mishnah permitted two things:
"Aval sachin" - one may smear fruit juice and the like onto the Pesach offering while it is roasting, and this is not considered cooking.
"U'matbilin oso bahen" - at the time of eating one may dip the roasted Pesach offering into a liquid, and this does not constitute drawing the roasted flavor out of it.
"Mei tashmisho shel nachtom":
Here the mishnah returns to the concern of chametz. It was customary for the baker to keep a vessel of water beside him, in which he would rinse his hands and smooth the dough. This water, the baker's working water, must be poured out and may not be used, since scraps of dough remain in it and it will become chametz if it is left standing where it is.
In summary: in this mishnah we learned two separate matters - the prohibition against putting flour into charoses and into mustard out of concern for leavening, and the distinction between mustard, for which eating it immediately is sufficient, and charoses, which becomes leavened especially quickly, as well as the opinion of Rabbi Meir, who is stringent even with mustard; and alongside them, the law of the Pesach offering, which may not be cooked either in liquids or in fruit juice, though it may be smeared with them while roasting and dipped into them at the time of eating. Finally we returned to the concern of chametz in the law of the baker's working water, which is poured out because of the scraps of dough in it.