Kelim, Chapter Eight, Mishnah Eleven
This mishnah deals with various liquids that come from an impure woman and defile an oven.
1) If milk [of an impure woman] dripped from a woman’s breasts and fell into the air-space of an oven, the oven becomes unclean, since a liquid conveys impurity regardless of whether one wanted it there or not.
2) If she was sweeping it out and a thorn pricked her and she bled, or if she burnt herself and put her finger into her mouth, the oven becomes unclean.
Section one: Liquids convey impurity whether or not one wanted them to end up where they end up. As we shall see, there are other purity laws concerning liquids that work differently, namely when does a liquid make a food susceptible to impurity.
In any case, assumedly the woman did not want her breasts to leak onto the oven. Nevertheless, if this impure milk (impure because she is impure) falls into the oven, it defiles the oven.
Section two: If she is sweeping out the oven and she pricks her finger and bleeds into the oven, her blood defiles the oven, even though she certainly didn’t want it there.
Similarly, if she burns herself and puts her finger in her mouth, covering it with spit, and then puts her finger back into the oven, the spit on her finger defiles the oven, even though she wanted the spit on her finger, not on the oven. In all cases, if an impure liquid enters an oven, it defiles it.