Keilim, Chapter 8, Mishnah 4. Throughout this chapter we have been occupied with the hollow interior of a clay oven and which items do or do not become tameh by being suspended within it. The present mishnah takes up a case in which that inner space has no effect at all, and then presents an unexpected twist.
The mishnah begins: "kedeirah shehi netunah batanur", a pot is sitting inside an earthenware oven, "hasheretz batanur", and a dead sheretz is found in that oven. The oven itself certainly becomes tameh. But what about the pot resting inside it? The mishnah rules: "hakedeirah tehorah", the pot remains tahor.
The reason is stated in the mishnah itself: "she'ein keli cheres metamei kelim", an earthenware vessel does not convey tumah to other vessels. This is the rule we have been building toward: a tameh earthenware vessel, such as the oven in our case, transmits tumah only to food (and to liquid), but not to people and not to utensils, whether they touch it or sit within its airspace. So a pot standing inside a tameh oven stays clean.
The Exception: A Little Liquid Changes Everything
Now comes the case in which the pot does end up tameh. The mishnah says: "hayah bah mashkeh tofei'ach". There is moisture inside the pot, and not merely a trace: enough of it that it could dampen another object, and that object would then be damp enough to dampen a third. Concerning such moisture the mishnah teaches "nitma vetimei'ah": first the liquid picks up tumah, and afterward the liquid passes that tumah on to the pot holding it.
Follow the chain. The liquid is sitting in the airspace of a tameh earthenware oven, and an earthenware vessel does convey tumah to food and liquid. So the liquid becomes tameh. Once the liquid is tameh, it renders tameh the vessel it is touching, namely the pot. That last step is a rabbinic decree; on a Torah level liquid has no power to make a utensil tameh.
Counting the Levels
Each act of transmission lowers the tumah by one rung. A sheretz carcass carries the status of av hatumah. Contact with it turns the oven into a rishon. The oven, in turn, gives the moisture inside the pot the status of sheini. Now, Torah law grants no utensil-defiling power to a sheini; only an av hatumah can render a utensil tameh. The Sages, however, instituted that liquid which is itself tameh imparts tumah to any utensil it touches, and it is by virtue of that enactment that our pot becomes tameh.
The Pot's Protest
The mishnah captures the oddity with a memorable line. "Harei zeh omer", as though the pot itself speaks up in astonishment to the liquid inside it: "metam'echa lo tim'uni, ve'atah timeitani", the very thing that made you tameh could not make me tameh, and yet you have made me tameh!
And that is exactly what has happened here. The oven, tameh though it was, could do nothing to the pot, because a clay vessel simply does not pass tumah on to utensils. The drops of moisture inside the pot, on the other hand, drew their tumah from that very oven and nevertheless do have such power, thanks to the enactment of the Sages that tameh liquids defile utensils. The result is a curious inversion: whatever made the liquid tameh has no reach into the pot, while the liquid it created reaches it easily.