Taharos, Chapter 3, Mishnah 3. This chapter deals with liquids emerging from tamei solids, and it rests on a basic rule: tamei food conveys tumah to a liquid only when the food amounts to a kabeitzah, an egg's volume, which is the minimum shiur for food to transmit tumah. Our mishnah applies that rule to a person crushing olives and grapes, and then draws a sharp contrast with the case of a zav.
A tamei meis who crushes olives and grapes
"Tmei meis shesochatz zeisim va'anavim." Here is a person carrying the highest grade of tumah, tumas meis, and he has olives and grapes before him. He does not simply take hold of them with his hands, for that would make the case too easy. Rather he presses and crushes them using flat wooden utensils, which themselves are not receptive to tumah. He has already rendered the fruit itself tamei; the oil and juice inside, as we find in the case of liquid held within a tamei solid, are not tamei.
"Kabeitzah mechuvan tahor": if the fruit in his hands measures precisely an egg's volume, or anything less than that, whatever liquid is squeezed out stays tahor. One condition attaches: "uvilvad shelo yiga", he must not himself make contact "bimkom hamashkeh", with the spot where the liquid has collected. "Yoser mikabeitzah, tamei": with more than a kabeitzah of fruit, the liquid does contract tumah, and the mishnah supplies the reasoning, "shekeivan sheyatzas tipah rishonah nitmes bekabeitzah." At the instant the opening drop separated from the fruit, a full kabeitzah of tamei food was still present, so that drop picked up tumah from it. Every drop that follows then derives its tumah from that first one.
This ruling parallels the case of tamei olives that release their oil, and it is placed here specifically as a foil for the case that follows, where we ask how the whole picture changes when the source of tumah is a zav rather than an ordinary tamei person.
When the presser is a zav
"Im hayah zav o zavah, afilu gargar yechidi, tamei." Now exchange the actor: in place of the tmei meis, put a man afflicted with a flux, or a woman in her state of zivah. A single solitary grape, a quantity nowhere near an egg's volume, will already yield liquid that is tamei, and immediately. Notice the difficulty in this: that lone berry stands below the shiur, so it has no power at all to pass tumah along to anything. What, then, defiles the juice? "Shekeivan sheyatzas tipah rishonah nitmes bemasa." As soon as the opening drop leaves the fruit, it acquires tumah by way of masa.
Here is the key to the whole mishnah. Until now, and largely from here on as well, we have been dealing with forms of tumah that are transmitted by maga, by touch. That is precisely the case with the tmei meis in the first half of this mishnah, and with the tumah discussed in the case of olives releasing their oil. A zav, however, also conveys tumah through masa, through carrying. So the drop of liquid that emerges under his hand is rendered tamei by the fact that he is carrying it, and not at all by the berry or other food item that had been holding it.
"Zav shecholav es ha'ez, hechalav tamei, shekeivan sheyatzas tipah harishonah nitmes bemasa." A second illustration seals the mishnah. Picture a zav drawing milk from his goat: what collects in the pail is tamei, and the reasoning is identical. The very first drop to leave the udder is defiled by masa, since in the act of milking the zav (or zavah) is bearing that drop.