Challah, Chapter 2, Mishnah 3. This mishnah deals with the difficulties that arise in separating challah, and with the question of how a person should act when he is unable to perform the separation properly.
Separating challah when one cannot recite the berachah:
It is a halachah that a person may not recite a berachah when he is not properly dressed, and especially when his nakedness is exposed. From this a problem arises: when one cannot recite the berachah, one also cannot separate challah.
On this matter the mishnah distinguishes between a woman and a man, based on the structure of their bodies:
The woman: she sits and separates her challah even while she is unclothed and has no garments on, since given the structure of her body, when she sits pressed against the ground the area that must not be exposed is not exposed. Therefore she may recite the berachah, and she may even separate challah under these circumstances as a first option.
The man: he cannot cover himself when he has no garments on merely by pressing against the ground, and therefore he may not separate in this manner.
One who cannot prepare his dough in purity:
From here the mishnah moves to an entirely different halachah: a person who is unable to prepare his dough in purity - whether he himself is tamei, or his vessels are tamei, or the flour became tamei - and in any case the dough is going to become tamei. The question is whether he should prepare dough in such a situation and separate challah from it, or not.
The opinion of the first tanna: it is better that he make his dough less than the required measure, less than the quantity that obligates challah, so that he will not come to a separation. The measure of obligation is five quarters of a kav, that is, a quarter of a kav more than a kav; therefore he should make batches of one kav at a time, less than the measure of obligation, and he will not separate challah in a state of tumah, since the challah is not meant to become tamei. It is better that the obligation of challah never take effect on it at all.
The opinion of Rabbi Akiva: it is better that he make his dough in a state of tumah, and not make it in single kav batches in order to exempt himself from the obligation of challah. For just as he calls the pure dough challah, so too he gives the status of challah to the impure one: both are called "challah laShem" - challah for Hashem, even the one that became tamei.
But when he makes batches of only a kav, "ein lahem chelek laShem" - they have no portion for Hashem, meaning no part of the dough goes up for the sake of the Holy One, blessed is He. It is better that there be a portion that goes for His sake, even though it is tamei. True, it cannot be eaten, but it can be given to a kohen, like impure Terumah, and the kohen will use it for fuel. This is permitted, and in Rabbi Akiva's opinion it is preferable to do so and to give the challah for Hashem's sake as one should, even though it is tamei, than not to do so at all.
In summary: in this mishnah we learned about two situations in which a person has difficulty performing the separation properly: when the berachah cannot be recited because of a lack of clothing - and here the mishnah distinguishes between the woman, who can cover herself by sitting on the ground, and the man, who cannot; and when the dough cannot be prepared in purity - and here the first tanna holds that he should make single kav batches and never come to the obligation, while Rabbi Akiva holds that he should prepare it in a state of tumah, since even impure challah is called challah for Hashem's sake, whereas with batches of a kav there is no portion at all that goes up for His sake.