Keilim, Chapter 8, Mishnah 11. This chapter has been occupied with the special sensitivity of a clay oven, which becomes tamei through its air space: a source of tumah that enters the hollow of the oven, even without touching its walls, renders the oven tamei. Our Mishnah presents additional ways in which an oven can come to be tamei, and it does so through a series of everyday scenes in a woman's kitchen.
Begin with the first scene: "Ha'ishah shenataf chalav midadeha venafal la'avir hatanur." Picture a woman who is herself teme'ah leaning over an oven that is still tahor, and a drop of her milk falls down into the empty cavity within it. Since she carries tumah, the milk that comes from her body is a tamei liquid. The Mishnah rules that the oven has become tamei.
A reason is attached: "shehamashkeh metamei lirtzon veshelo lirtzon." A liquid that is tamei passes its tumah along to whatever it reaches, and it is irrelevant whether anyone was pleased to have it arrive there. Desire and consent simply do not enter the calculation. Obviously this woman had no wish for milk to escape her, yet that changes nothing: the drop that landed inside the oven's hollow leaves it tamei.
Two more pictures of the same kind follow. "Haytah gorfato vehikatah kotz veyatza mimenah dam": she is clearing the ashes out of the oven's interior when a thorn jabs her hand, and blood now flows inside that hollow. "O shenikhvah venatnah etzba'ah letoch pihah": alternatively, she is scorched while tending the oven, sucks the burned finger to soothe it, and puts her hand back to work, so that a wet finger now sits inside the oven's empty space. In each picture the woman is teme'ah, so the blood and the moisture from her mouth carry the status of tamei liquids.
For both scenes the verdict is "nitma": the oven contracts tumah, in one instance from her blood and in the other from her spittle. Once again nobody asks whether she wanted her hand pricked or her finger moistened. Tamei liquid does its work regardless.
The thread running through the entire Mishnah is therefore a single principle applied to three homely situations: liquids that are tamei do not depend on a person's will in order to convey tumah, and once such a liquid reaches the hollow of a clay oven, the oven that had been tahor is tamei.