Maseches Ma'asros, chapter three, mishnah 8. Already in mishnah 5 we established that a courtyard is able to function like a house: when a person brings produce into the courtyard, his intention is to store it there, and from the moment it enters the courtyard the obligation of ma'aser takes effect. This ruling applies only to courtyards that possess that essential quality of a house. But just as a house obligates in ma'aser only from the moment the produce passes through the entranceway - this is "seeing the face of the house" - so too in a courtyard: only from the moment the produce enters through the entrance, or its equivalent, does the obligation take effect.
The case in the mishnah:
The mishnah discusses a person who has a fig tree growing inside his courtyard. In such a case, the fruit of that tree - those figs - will never see the face of the courtyard, since they are inside it from the very start. The question, then, is when the obligation of ma'asros takes effect upon them.
Why the mishnah chose specifically a fig tree:
The completion of processing is ambiguous: figs can be eaten fresh, and they can also be eaten dried or pickled.
The nature of the harvest is ambiguous: with most fruit trees the harvest is a one time event - the mishnah calls this a single gathering, one collection of the fruit. With a fig tree, by contrast, the harvest is drawn out and continuous. It is precisely this ambiguity that makes possible the entire discussion the mishnah wishes to clarify.
And so the mishnah says: "Te'einah shehi omedes bechatzer" - a fig tree standing in a courtyard, and we are speaking of a courtyard such that had produce been brought into it through its entrance, it would have become obligated in ma'asros; only that here this will never happen, since the fruit is inside the courtyard from the very start. Therefore "ochel achas achas u'patur" - a person may eat in that courtyard one fig at a time, and he will never become obligated in ma'aser, since the courtyard has no effect on the obligation here.
The background learned in the second chapter:
The usual process by which produce grows and the obligation of ma'aser takes effect is as follows: after the completion of processing, if he intends to sell the produce, the obligation of ma'aser takes effect already at that point; and if he intends to bring it home, the obligation does not take effect until he brings it home through the entrance into the courtyard. We also said that there are actions a person performs that have the power to advance the onset of the obligation, such as separating Terumah Gedolah, cooking or salting.
One of the examples brought there was a sale. A person who buys figs is not going to do any further processing on them, and so this is a kind of completion of processing. The mishnah brought interesting cases there: one who buys five figs from a seller - this is like a miniature harvest. By Torah law, as long as all five figs have not been picked from the tree, that tiny harvest has not been completed and there is no Torah obligation. But rabbinically, from the moment he begins to accumulate purchased figs, that accumulation is called tzeiruf - the gathering together of a number of figs - and at that point the obligation to tithe takes effect, at least rabbinically.
What is 'tzeiruf':
Something very similar happens in our mishnah. In the courtyard, each individual fig is exempt from ma'aser, since it did not pass through the entrance, and this is casual eating. But once there is tzeiruf - once the person has begun to gather several figs together in the courtyard - and since it is not at all clear to a third party what is going on here, the Sages obligated him to tithe the collection of figs from the moment he holds two or more, according to the Talmudic comment.
And this is the language of the mishnah: "Im tzeiref - chayav". Once he holds two figs that he picked from the tree, he is obligated - at least rabbinically - to tithe the figs in the courtyard before he continues eating them.
The view of Rabbi Shimon:
Rabbi Shimon disagrees, and apparently his disagreement centers on the very definition of gathering together (tziruf), both here and in other places. According to him, "achas biyemino ve'achas bismolo ve'achas befiv" - this is not a collection of figs but three separate figs, each distinct from the other, and at this point no gathering has taken place. In other words, according to Rabbi Shimon a person may hold, even initially, one fig in one hand, a second in the other, and a third in his mouth, and no obligation to tithe applies to him, since this is not called gathering. The halachah, however, does not follow Rabbi Shimon, but rather the Talmudic observation: any two figs held in one's hand at one and the same time, even in two different hands, are considered gathered together and become obligated in maaser within the courtyard.
"Alah lerosho":
The Mishnah continues: if the person climbed up into the fig tree, he has left the domain of the courtyard and entered a separate domain - the covering of the ground formed by that tree. Therefore "memaleh es cheiko ve'ochel" - the obligation to tithe when one gathers more than one fig at a time does not apply here; rather, he may fill his lap. "Cheik" is the fold of a garment: one who wears a cloak forms in it a kind of pouch, and that is called his cheik. He fills it with figs and eats them, and even though he gathered two or more in a single gathering, since he has left the courtyard and his eating is a casual eating, he may eat them with no obligation to tithe at all.
In summary: In this Mishnah we learned that a fig tree standing inside the courtyard never sees the face of the courtyard, and therefore one eats from it one at a time and is exempt. From the moment he gathers two figs or more, he is obligated by rabbinic law to tithe, while Rabbi Shimon, whose view is not the halachah, narrows the definition of gathering. And if he climbed to the top of the tree, he has left the domain of the courtyard, and he fills his lap and eats, exempt.