Eruvin, Chapter 10, Mishnah 6. The Mishnah opens with the law of drinking when a person's body is in one domain and his mouth is in another:
"Lo ya'amod adam bir'shus hayachid v'yishteh bir'shus harabim" - a person may not stand in a private domain and drink in a public domain, that is, when part of his body is in the public domain and the drinking takes place there.
And likewise the reverse: one who stands in a public domain "v'yishteh bir'shus hayachid" - and drinks in the private domain, when his head and mouth are inside the private domain.
"Ela im kein hichnis rosho v'rubo l'makom shehu shoseh" - unless he brings in his head and the greater part of his body. It is not enough to put in his mouth alone; he must bring his head and the majority of his body into the domain in which he is drinking.
The reason for this:
The concern is that the person may want to take along the vessels he is drinking from and bring them into the domain where the rest of his body is. When the majority of his body is in the domain where he is drinking, there is no such concern; the concern exists only when the majority of his body is situated in one domain and the drinking takes place in another, and then we are worried that he will pick up the vessels and bring them into the domain where his feet are.
"V'chein bagas" - and likewise in the winepress:
The Gemara explains that this law does not relate to Shabbos, but to the laws of maaser. Produce whose processing has not been completed may be eaten or drunk from in a casual manner, and not as a set meal, even though Terumah and maaser have not yet been separated from it. Drinking in the winepress is considered casual drinking, and it is therefore permitted before the separation is made.
The Mishnah is teaching, then, that the same rule applies here as well: the head and the majority of the body of the one drinking must be located within the area of the winepress, so that the drinking should count as casual drinking. But if the majority of his body is outside the winepress, the act takes on the status of drinking outside the winepress, which is not casual drinking but a set drinking, and Terumah and maaser must first be separated.
Mazchilah and tzinor:
Here the Mishnah changes direction and deals with a person who wishes to drink from the rainwater coming off a roof, distinguishing between a mazchilah and a tzinor:
Mazchilah: the gutter that runs along the length of the roof, positioned in such a way that one who drinks from it drinks from a section that is within three tefachim of the roof itself.
Tzinor: the spout that extends out of the mazchilah and is distanced from the building, more than three tefachim from the roof.
This distinction is the foundation for the laws that follow. The Mishnah rules that anyone may drink "min hamazchilah l'matah mei'asarah tefachim" - from the mazchilah below ten tefachim, even though the public domain is below ten tefachim and he is standing in it at the moment he takes the water. This is because he is not drinking directly from the gutter, but rather catching the water from the stream as it descends through the air, whether into a vessel or with his mouth as it comes down.
The reason he may not drink from the mazchilah itself: the mazchilah, since it is within three tefachim of the roof, is considered a private domain, and so he would be transferring water from a private domain to a public domain. He may therefore only catch the water in mid-air, and in doing so he is not taking from a private domain but from within the public domain itself.
But the Mishnah continues: "Umin hatzinor mikol makom shoseh" - and from the tzinor he may drink from anywhere. The tzinor, which is more than three tefachim away from the roof and less than four tefachim wide, has the status of a makom petur, an exempt area. He may therefore drink from it from any direction, and may even put his mouth right up against it, for although he is standing in a public domain, it is permitted to take from an exempt area into a public domain.
In summary: in this Mishnah we learned that a person may not drink when the majority of his body is in one domain and his mouth is in another, out of concern that he will bring the vessels after his body; and that the same rule applies in a winepress with regard to casual drinking, which is exempt from the separation of Terumah and maaser. We also learned the distinction between a mazchilah, which has the status of a private domain and from which one may drink only by catching the water in the air, and a tzinor, which has the status of an exempt area, and from which he may drink from anywhere.