Taharos, Chapter 10, Mishnah 5. We are in the middle of the chapter that deals with grapes and the winepress, and our mishnah turns on one basic distinction: what were these grapes designated for? Grapes that a person set aside in order to eat them are in one category, and grapes that were gathered in order to be pressed into wine are in another. The juice that leaves them is treated very differently in each case.
Eating grapes: the juice does not matter
The mishnah begins with the words "Ha'ochel min hasalim umin hamishtach shel adamah". Two situations are described: a person eating grapes that are sitting in the baskets, and a person eating grapes taken from a stretch of bare earth prepared for laying grapes out to dry. In each situation the fruit had been earmarked for eating rather than for the press.
What happens if those grapes get crushed in the process? "Af al pi shemevuka'os umenatfos lagas", even if they are split open and their juice is dripping down into the winepress, "harei hagas tehorah", the winepress remains tahor. Since these grapes were designated for eating, the liquid coming out of them is not liquid that a person wants, and therefore it does not prepare the produce to receive tumah in the first place. Juice that nobody is interested in simply does not do the job.
Pressing grapes: one grape that falls in
The mishnah now presents the reverse situation: "Min ha'avas umin hamishtach shel alim", grapes taken from the big vat, or from ground spread over with leaves. Fruit in these places was designated for pressing, so the juice it gives off is fully significant. The case is that "venafal mimenu gargir yechidi": out of that quantity, one solitary grape drops into the gas.
The outcome depends on the condition of that one grape. "Im yesh lo chosam, tahor", if the grape still has its stem attached, it is considered sealed, and it is tahor. The presence of the stem tells us that no juice has seeped out through the skin, so nothing has happened that would render the winepress tamei. "Ve'im ein lo chosam, tamei", but if the stem is gone, it is tamei, because in that case even a tiny drop of juice escaped, and that drop was enough to bring tumah to the whole.
Grapes trodden on with tamei hands
The last case: "Naflu mimenu anavim uderachan bimkom hamufneh". A person is eating grapes with hands that are tamei, some grapes fall from him, and he then treads on them in a cleared, empty spot. Here again there is a distinction, and it is a formulation we have met before in these mishnayos.
"Kabeitzah mechuvan, tahor", if there is exactly a kabeitzah of grapes, no more and no less, it is tahor. The reason is precise: the very first drop of juice that leaves the grapes reduces what remains to less than a kabeitzah, and less than a kabeitzah of food cannot impart tumah to anything else.
"Yoser mikabeitzah, tamei", when the quantity exceeds a kabeitzah, the result is tumah, "keivan sheyatz'ah tipah rishonah nitmeis bekabeitzah". Here, once that initial drop has come out, a complete kabeitzah of grapes is still sitting there, and that kabeitzah is enough to render the drop tamei. The tamei drop then transmits tumah to the remaining liquid, and each drop that emerges from then on is tamei too.
So the whole mishnah rests on two measurements that we might easily overlook: what the owner had in mind when he set the grapes aside, and whether a kabeitzah remains after the first drop leaves.