In Masechet Menachot, Perek Hei, Mishnah Aleph, the Mishnah says: All of the Menachot (meal offerings) are brought as matzah. What we mean by matzah is not the thin type of cracker that we're talking about, but a type of bread that is not chametz (leavened dough). It could be thick, it could be thin, it could be fried or baked, but it's not the conventional matzah that we know. So all the Menachot were brought as matzah.
Chutz me'chametz sheba'todah u'shtei halechem she'hen ba'ot chametz. Except for the loaves of the todah offering. The todah offering was brought with four sets of ten loaves, so forty loaves total, and each one of those sets was prepared in a different way. Three of them were matzah, one of them was chametz, and the u'shtei halechem, the two loaves that are brought as a Mincha (meal offering) on Shavuot, she'hen ba'ot chametz, both of these are brought as chametz.
The question becomes: How were the loaves of the todah and the u'shtei halechem turned into chametz? Normally, if a person had dough and wanted to turn it into chametz, they would add in a little bit of sourdough. Sourdough is very fermented dough that has the ability to cause dough that it's mixed with to rise. So in making these Menachot, the problem is that lehalacha (according to Jewish law), you're not allowed to have more than one issaron (one-tenth of an ephah) of flour. So if sourdough was added, you end up with a different number. You don't end up with more than an issaron. So therefore, it can't be that they added sourdough to the mix in order to turn them into chametz.
So what they do, it says to Rabbi Meir, what they would do is, after the person mixed the issaron of flour for each loaf, they would take some of that flour, mix it with water in order to make dough, and allow it to ferment on a very high level and become sourdough. Then they would take that sourdough, mix it back into the Mincha, and turn it into chametz. So no extra flour was needed in order to create the chametz.
Rabbi Yehuda omer, says to Rabbi Meir, even a Mincha offering made this way is not ideal because sourdough is not going to ferment fully, and it's not going to make the Mincha rise like it should. The flour of the Mincha was already measured out properly. So the owner was not going to allow it to sit out for too long, because he doesn't want it to become tameh (ritually impure), and then he can't bring it as a korban (offering). So the flour that he's removing is not going to have really enough time to turn into sourdough.
So, says Rabbi Yehuda, what does the person do? He should bring fully fermented sourdough, put it into an empty measuring bowl that contains the amount of one issaron, and then fill the rest of the measuring bowl with flour from the Mincha. So the sourdough is going to cause the Mincha to rise fully. And it's also not going to be more than an issaron, because you're taking the flour of the Mincha and adding it to the bowl that has the sourdough already there. So the volume of the sourdough was already taken care of, and it's sourdough that was fermented properly. It's not like it's done quickly. So therefore, that's a better solution according to the opinion of Rabbi Yehuda: fully fermented sourdough that's going to create a full issaron, and it'll make the Mincha turn into a full issaron.
Rabbi Meir is going to respond. Rabbi Meir said to Rabbi Yehuda, even a Mincha made with this method is going to have an imprecise measurement of flour: either too little flour or too much sourdough doesn't have the same volume as regular flour. It's already risen. It's already something else. It's mixed with water. It's fermented. And therefore, the volume is going to be different than when you have flour. So therefore, if you put the sourdough in a bowl and add flour to it, and you think it's going to be an issaron, in reality, it's not, because of the different way that the volume works with the sourdough. So therefore, it's going to be an imprecise amount.