Menachos, Chapter 5, Mishnah 2. The Mishnah continues to deal with preparing the dough of the menachos - both the fried ones (the machavas and the marcheshes) and the baked ones (the challos and the rekikim). These are four of the five types of the voluntary minchah offering, and the Torah forbids them to become chametz.
Kneading the dough with lukewarm water:
How are these doughs prepared? The verse mentions mixing with olive oil alone, but clearly a small amount of oil does not turn flour into dough. Water must be added - and it is the water that creates the possibility that the dough will become leavened. Therefore we learned: "Kol hamenachos nilashos b'poshrin" - with lukewarm water, roughly room temperature, neither cold nor hot. With this water one prepares the dough from which the baked or fried minchah will be made.
The commentators disagree about the order of preparation: some understand that the oil is added first (this is how the Rema understands it), and some understand that the dough is first made from flour and water alone, and only afterwards is the oil added (this is how Rashi understands it). In any case, the essential point is the use of water that is neither too hot nor too cold:
Hot water: it scalds the flour, and such scalding is a form of cooking, so the minchah is disqualified. Even after the fact it is entirely invalid.
Cold water: the dough does not hold together well and forms clumps, which is not the ideal way; but after the fact it is valid.
"U'meshamran shelo yachmitzu":
Once the flour has been mixed with water, its law is like that of dough made on Erev Pesach: it is incumbent upon the one preparing it to make sure it does not become leavened. How? Through constant working of the dough. The halachah is that as long as the dough is being rolled, worked and moved from its place, it cannot become leavened. Even if he mixes the water into the flour for three hours, as long as he continues to knead and mix it, it will not become chametz; but without such working, after eighteen minutes the dough is considered chametz.
Leavening of the leftovers:
"V'chi timtza shiureha over b'lo saaseh" - not only is the dough forbidden to become leavened before frying or baking, but even the leftovers - the portion that is never cooked and never baked but is eaten - if they become chametz, one transgresses a negative commandment, a prohibition from the Torah.
An important point that the Mishnah does not state explicitly: the discussion here is about the fifth type of voluntary minchah - the minchas soles, which is offered raw, without baking and without frying. For with the fried and baked menachos, once the dough has been cooked it can no longer become leavened: the kometz is offered, and even if the leftovers remain - the machavas, the marcheshes, the challos and the rekikim - they will not become leavened any more. But the minchas soles is never baked, and therefore it is liable to become leavened. How? Even though it is made only of flour and oil, water may drip onto it, and if water fell onto the leftovers, they are liable to become leavened. This is a separate prohibition, apart from the portion offered on the altar.
The next words in the Mishnah, "shene'emar", refer back to the first law in our Mishnah: the prohibition against allowing dough that is destined to be offered on the altar to become leavened, and not to the leftovers that are eaten afterwards. And this is the language of the verse: "Kol haminchah asher takrivu laHashem lo se'aseh chametz" - any minchah that is offered to Hashem, that is, upon the altar, may not be allowed to become leavened.
Three separate prohibitions:
The Mishnah teaches that the prohibition of allowing dough to become chametz is one matter, while performing work with that chametz dough is a prohibition in its own right. There are thus three stages: kneading, shaping and baking, and each one carries its own separate prohibition. Therefore, in practice, one who does this deliberately and after proper warning is liable to lashes, and it is possible for him to become liable to three separate sets of lashes: for the kneading, for the shaping and for the baking.
Kneading: mixing the flour and water together. If he did this with a thick chametz dough, this is the first prohibition.
Shaping: according to some commentators this is the kneading itself; according to others, kneading means kneading in its plain sense, while shaping refers to arranging the loaves - braiding the challah and the like, forming it into loaves ready to be baked.
Baking: the actual baking, which is explicitly forbidden in a separate verse.
One verse says "lo se'aseh chametz" - do not let it be made into chametz; and a separate verse says "lo se'afeh chametz" - do not bake it as chametz. From the fact that there are two verses we learn that each one is a prohibition in its own right, and as many as three separate prohibitions arise for a person who mixes the flour with water, kneads, shapes and bakes it as chametz.
In summary: in this Mishnah we learned that all the menachos are kneaded with lukewarm water - not with hot water, which scalds the flour and disqualifies it, and not with cold water, which does not bind the dough together; that one must guard them from becoming chametz by working the dough continuously; that even the remainders that became chametz, and especially in the flour offering that is not baked, carry a prohibition of their own; and that the acts of kneading, shaping and baking chametz involve three separate prohibitions, derived from the two verses "lo se'aseh chametz" and "lo se'afeh chametz".