Menachos, Chapter 6, Mishnah 4. The Mishnah now turns to pesisah - breaking the loaves of bread into pieces, which is done before pouring on the remainder of the oil and before taking the kometz (handful).
Which meal offerings require pesisah:
The opening statement of the Mishnah comes to clarify which meal offerings are obligated in pesisah: "Kol ha'menachos ha'na'asos bi'kli - te'unos pesisah" - all meal offerings that are prepared in a vessel require breaking into pieces before the kemitzah.
This wording is somewhat misleading, since it does not mean what it meant in the previous Mishnah. The intent is that there are meal offerings prepared in an oven, which according to this Tanna is not considered an earthenware vessel, and those are exempt from pesisah - namely the lechem ha'panim (the twelve loaves of the showbread) and the shtei ha'lechem brought on Shavuos. But all the other meal offerings do require pesisah.
How the pesisah is done:
The verse says "Pasos osah pitim" - crumble it into pieces. The understanding of the verse is that "pasos" refers to one breaking and "pitim" to a second breaking: one takes the loaf and folds it in two, and then folds it again into four. If it is brittle, it naturally breaks into pieces and crumbs; but if it is like a pancake made from a batter, such as the machavas offering prepared in a deep pan, it may fold without tearing along the creases, and then there is a technical problem that it has not actually been broken, and one must break it further.
The wording of the Mishnah:
"Minchas Yisrael - kofel echad li'shnayim u'shnayim le'arba, u'mavdil" - in a meal offering brought by a Yisrael (as opposed to a kohen), one folds the loaf in two, and the two into four, and if it did not break along the creases one actually tears it apart, so that there will be smaller pieces.
"Minchas kohanim - kofel echad li'shnayim u'shnayim le'arba, ve'lo mavdil" - here too pesisah must be performed, and one separates it twice by folding, but there is no need to actually break it if it did not break on its own. The essence of pesisah is the folding component and not the tearing component, and since the meal offering of a kohen is burned entirely and no kometz is taken from it, there is no need to actually separate it.
The meal offering of the anointed kohen - "lo hayah mekaplah" - the chavitin that the Kohen Gadol brings each day do not require folding at all. These too are twelve loaves, half in the morning and half in the evening, and there is no kemitzah in these or in those.
The opinion of Rabbi Shimon:
Rabbi Shimon says: "Minchas kohanim u'minchas kohen mashiach ein bahen pesisah, mipnei she'ein bahen kemitzah, ve'chol she'ein bah kemitzah ein bah pesisah" - that is, no pesisah is done at all with the meal offering of a kohen and with the chavitin of the Kohen Gadol, since no kometz is taken from them, and pesisah and kemitzah are bound up with one another.
The basis of this is Rabbi Shimon's approach of expounding the reason behind the verse: in his view the pesisah is done for a particular purpose - in order to obtain the pieces for the sake of the kemitzah. The Sages disagree with him and do not attribute reasons to the verses: the verse said to break, and therefore one breaks, whether it is for the sake of kemitzah or not for the sake of kemitzah. And the halachah follows the Sages in this, that the meal offering of a kohen is folded as well.
"Ve'kulan ke'zayis":
The Mishnah continues: "V'chulan k'zayis" - "and all of them are the size of an olive." There are two ways to understand what this refers to:
It is a further statement of Rabbi Shimon: first, wherever no kometz is taken there is no need for crumbling at all; and second, wherever there is an obligation to crumble, dividing into quarters is not enough, and one must break the pieces down to the size of an olive.
"V'chulan k'zayis" is the statement of the first Tanna, that is, the Sages, who say that when folding, one folds until the pieces are the size of an olive.
The Bartenura follows the approach of the Rambam in his Commentary on the Mishnah, that this is a general statement referring back to the first Tanna, except that in his view the picture of the crumbling itself is different: first one breaks the pieces of the minchah down to the size of an olive, and then one takes those olive-sized pieces and divides them into quarters. It emerges that the crumbling takes place after the piece is already the size of an olive, and not before.
It is worth noting that although the Rambam says this in his Commentary on the Mishnah, he does not say so in the Mishneh Torah. In the Mishneh Torah he rules as a matter of halachah that one divides into quarters without the element of an olive's size, and it is enough that the pieces be small enough that a kometz can properly be taken from them, with no need to break them down into tiny pieces the size of an olive. And so is the halachah in practice: one divides into quarters only.
If a kometz is going to be taken from the minchah, one must actually separate the pieces and take the kometz from it. And if what comes up in the kohen's hand is too much, because he took a large piece of bread, he scrapes off the excess with his thumb and his little finger, until exactly a full measure of the three middle fingers remains in his hand - and according to the Rambam, four fingers - and that is the kometz.
In summary: In this Mishnah we learned that all menachos prepared in a vessel require crumbling, excluding those baked in an oven - the lechem hapanim and the shtei halechem. We saw the manner of crumbling derived from "pasos osah pitim": folding in two and in four, and the difference between the minchah of a Yisrael, in which the pieces are actually separated, the minchah of kohanim, in which they are not separated, and the minchah of the anointed kohen, which is not folded at all. We also learned the dispute between Rabbi Shimon and the Sages regarding the reason behind the verse - where the halachah follows the Sages - and the two ways of understanding "v'chulan k'zayis," and that in practice dividing into quarters is enough.