Machshirin, Chapter 6, Mishnah 5. The mishnah immediately preceding ours listed the seven liquids, the Shiva Mashkin, that give food the ability to contract tumah. Those seven are the avos, the head categories. Now the mishnah proceeds to the toldos, the offshoots: fluids that are not literally water, blood, milk or oil, and yet fall under one of those headings and therefore accomplish hechsher exactly as the primary liquid does.
The Derivatives of Water
Our mishnah opens with the category of water and what branches off from it, "toldos hamayim." Included here are the fluids that a person's own body secretes. "Hayotzei min ha'ayin" refers to whatever issues from the eye; the mishnah goes on to what comes out "min ha'ozen," from within the ear; then "min hachotem," the moisture of the nose; and "min hapeh," saliva from the mouth.
The list continues with "mei raglayim," liquid waste, about which the mishnah specifies "bein gedolim bein ketanim." The Rav takes the two terms as covering both forms of elimination: a bowel movement that comes out in liquid form, and urine. By whichever route such liquid leaves the body, it belongs to the category of water.
And the mishnah adds a crucial qualifier: "lada'ato shelo lada'ato," whether the person released the fluid on purpose or it came out without his intending it. Unlike other areas of hechsher, where intent plays a decisive role, here it makes no difference. Any of these fluids, deliberate or not, is a hechsher l'kabel tumah.
The Derivatives of Blood
Next: "toldos hadam," the offshoots of blood. First, "dam shechitah babeheimah uvachayah uv'ofos hatehorim," the blood that pours out at the moment of shechitah. Anyone who has witnessed a shechitah knows how much blood is released. Whether it comes from a beheimah, a domesticated animal, from a chayah, a wild animal, or from a kosher bird, that blood counts as blood for our purposes.
Second, "v'dam hakazah lishtiyah," blood drawn by bloodletting when the purpose is drinking. Bloodletting was practiced, and the blood taken was at times given to a non-Jew or to an animal to drink. Since it is designated for drinking, it takes its place among the Shiva Mashkin and can effect hechsher l'kabel tumah.
Whey and Olive Liquid
"Mei chalav k'chalav," whey has the status of milk. Whey is the watery liquid that separates out from the milk when the curds form in the making of cheese, and the mishnah rules that it is treated exactly as milk itself.
"V'hamochal k'shemen": mochal has the din of oil. Mochal is the watery fluid that is squeezed out of olives in the press alongside the oil itself. Rabbi Shimon rules that its status is that of oil, and he supplies his reason: "she'ein hamochal," this olive water never "yotzei midei shemen," comes out entirely clear of oil. In strict terms, such watery runoff is not one of the seven at all; standing alone it would rank as ordinary fruit juice, which cannot bring about hechsher. But because droplets of oil are inevitably mixed into it, according to "divrei Rabbi Shimon" it invariably carries oil's status and is machshir l'kabel tumah.
Then comes the dissenting opinion, "Rabbi Meir omer." Rabbi Meir sets that reasoning aside and rules more expansively: "af al pi," even in a case "she'ein imo shemen," where no oil whatsoever is present, mochal in its own right is counted among the seven liquids and brings about hechsher. The halachah is in accordance with Rabbi Meir.
The Blood of a Sheretz
The mishnah closes with a striking exception: "dam hasheretz mitamei v'eino machshir, v'ein lanu kayotzei bo." A sheretz is one of the creeping creatures listed in Vayikra. Its blood carries the same tumah as its flesh, so it does convey tumah. Yet it does not effect hechsher; it does not render food susceptible to tumah.
"V'ein lanu kayotzei bo," and we have nothing else comparable to it. This is the single case in the entire system of a liquid that is itself a source of tumah and nevertheless lacks the power to make food susceptible to tumah.