Keilim, Chapter 9, Mishnah 4. This chapter deals with tumah that finds its way into the air space of a tanur, a clay baking oven, and with the question of when something that carries tumah inside of it is still able to transmit that tumah to the oven around it. Our mishnah takes up a case where the tumah is not visible at all: liquids that have been soaked up and hidden away inside an absorbent object.
The mishnah begins: "Sfog shebala mashkin teme'in venaguv mibachutz." A sponge drank up tameh liquids, and its outer surface is dry. Nothing can be seen or felt: the liquids sit entirely within the body of the sponge, and to the touch it is dry on the outside. "Venafal la'avir hatanur, tameh." This sponge then fell into the air space of a tanur, and the oven becomes tameh.
Why should the oven become tameh, when the tameh liquids never touch it? The mishnah gives the reason: "shesof mashkeh latzet," because in the end the liquid is destined to come out. A sponge is used precisely for this purpose: one soaks up liquid with it and then squeezes it out again. The liquid that entered it was never absorbed in the sense of losing its identity. It did not cease to exist inside the sponge; it is lodged there temporarily and will emerge exactly as it went in. Since those liquids are tameh, and they are regarded as still being present as liquids, they render the oven tameh.
Two arguments for purity, and why they fail
It would have been reasonable to suggest a different outcome: since the liquid has been drawn deep inside the sponge and is concealed there, perhaps concealed tumah cannot reach out and affect anything beyond it. That idea, however, belongs exclusively to a live creature that has ingested something tameh. In such a case what sits inside the animal is sealed within it and leaves the outside world tahor. A sponge is no creature, and the halachah of an animal's innards has no bearing whatsoever on it.
One might also have wanted to argue that the liquids are in a "tocho shel tocho," an inner space within an inner space, a situation we deal with elsewhere in which the oven remains tahor. But that leniency belongs to a case where the tumah lies inside a vessel whose rim projects above the mouth of the oven, so that the inner vessel is not truly within the oven's air space. Here the sponge is lying completely inside the oven. The sponge is inside, the liquids are inside the sponge, and the oven is therefore tameh.
A turnip and a reed
The mishnah extends the ruling to other materials that soak up liquid: "vechen chatichah shel lefet veshel gomi," and the same applies to a chunk of turnip or a piece of reed grass that has taken in tameh liquids. It was common practice to dip a wedge of turnip or a stalk of reed into a cask of wine and draw up a little of it for sampling, and afterwards such a piece could appear and feel perfectly dry on the surface. Even so, if it drops into the oven's air space, the tameh wine held within it makes the oven tameh, since that wine, too, will in the end come back out.
The view of Rabbi Shimon
A second Tanna does not accept the analogy: "Rabbi Shimon metaher bishnei elu." With regard to the turnip and the reed grass, Rabbi Shimon rules the oven stays tahor, because to his mind they are nothing like a sponge. A sponge holds no moisture of its own, so wine that enters it remains a guest, distinct from its host, and will leave exactly as it arrived. Turnip and reed grass, though, are themselves full of sap. Wine drawn into such a piece merges with the sap already present, batel within it, forfeiting any independent standing. Whatever moisture may later seep out is not the tameh wine reappearing but a blend into which that wine has been absorbed and lost. Since nothing tameh is slated to come out in its own right, Rabbi Shimon holds that the liquid hidden inside them has no power to make the oven tameh.
The mishnah thus leaves us with a fine but far reaching distinction: whether hidden liquid still counts as liquid depends on whether it is merely being stored, waiting to be squeezed out, or whether it has genuinely become part of the substance that took it in.