Niddah, Chapter 8, Mishnah 3. This chapter deals with a woman who discovers a bloodstain (a kesem) on her garment, and with the question of when that stain renders her tmeiah and when she remains tahorah. Our mishnah continues that discussion by listing the various explanations a woman is permitted to offer for the stain she has found.
The mishnah opens with a broad principle: "Vesolah bechol davar shehi yecholah litlos." She may attribute the stain to anything to which it can reasonably be attributed. In other words, if there is a plausible outside source for the blood, she is not required to assume that it came from her own body.
Sources she may point to
Concrete illustrations follow. "Shachatah behemah", "chayah va'of": having slaughtered a beast, an undomesticated animal, or a fowl, she had blood around her, and the mark discovered on her clothing can be laid at that door. "Nis'askah bechesamim": if her hands were busy with bloodstains, working with them and moving them about, that occupation likewise supplies a source for the mark. "O sheyashvah betzad ha'asukim bahen": the leniency extends even further, for she need not have been the one at work; it is enough that she was sitting alongside others who were handling such stains. Throughout these situations she has the right to conclude that what she found is not blood that came out of her, and consequently the stain leaves her tahorah.
Another instance: "Hargah ma'acholes." A louse that she crushed produces blood, and the mishnah rules "harei zo tolah bah", the mark on her clothing gets assigned to that killing.
How large a stain may be attributed to a louse
Now comes a question of measurement: "Ad kamah hi tolah?" How big can a mark be and still be accounted for in this way, instead of being traced back to her? Rebbi Chanina ben Antignos fixes the limit: "ad kegris shel pol", a gris, which is the bulk of a bean. There is a reason behind that particular shiur: it matches how much blood a single louse holds.
The mishnah then adds the words "ve'af al pi shelo hargah." Certainty is not required here. She may have no knowledge at all of any louse being squashed on her clothing, yet a mark within that shiur is still presumed to have come from one that she pressed or sat down upon unawares. Anything up to a bean's bulk, therefore, is not a mark that forces her to declare it dam niddah; she is free to credit it to a crushed louse.
Attributing the blood to someone else, or to her own wound
"Vetolah bivnah o beva'alah." Her child and her husband are also available as explanations, and so is any other member of the household, on condition that the person in question is carrying a wound. Once the blood might just as well have originated with one of them, the stringent assumption is not forced upon her.
The last case is an injury on her own body: "Im yesh bah makah", she has a lesion, "vehi yecholah lehigalot ulehotzi dam", one that is apt to open up and let blood out. In such a case the mishnah concludes "harei zo tolah bah": the mark she discovered is charged to that lesion, and she is not compelled to relate to it as dam niddah.
The thread running through the entire mishnah is that a kesem is not the same as blood seen emerging from the body. Where a reasonable alternative explanation exists, whether an animal that was slaughtered, blood she was handling, a louse she may have crushed, a household member with a wound, or a wound of her own, she leans on that explanation and remains tahorah.