Mikvaos, Chapter 9, Mishnah 1. This perek is devoted mainly to the halachos of tevilah and of chatzitzah. A chatzitzah is anything that comes between the water of the mikveh and the person or the utensil being immersed, forming a barrier that keeps the water from reaching the body or the object itself.
Where does this halacha come from? The Torah writes concerning tevilah: "verachatz es besoro bamayim vetaher," he shall wash his flesh in water and become pure. The Gemara (Sukkah 6a) learns from this pasuk that when a person or a keli is immersed, nothing may intervene, nothing may block the mikveh water from reaching the one who is toivel or the item being immersed. The Gemara adds a nuance drawn from the precise wording. The pasuk could have said "verachatz besoro"; instead it inserts the apparently superfluous word "es." That extra word broadens the halacha to include whatever is tafel lebesaro, whatever serves as an extension of the flesh, his hair being the primary example. Hair, then, falls under the rules of chatzitzah no less than skin does, so that at the time of tevilah a barrier is disqualifying whether it rests upon the body or sits within the hair.
The two conditions of a chatzitzah
Halacha LeMoshe MiSinai teaches that mide'oraisa, two requirements must both be present before something is judged a chatzitzah. Requirement one: the substance must spread over rov haguf, the greater part of the body, or over rov hase'ar, the greater part of the hair. Requirement two: it must be something concerning which the person is makpid, something he objects to having on him, something he would be embarrassed to be seen wearing or would not tolerate remaining on the utensil he is immersing.
The Chachamim broadened the halacha in two directions. They ruled midederabbanan that a covering spread over the greater part of the body invalidates the tevilah, and this holds true even when the person has no objection whatsoever to it being there. They also ruled that a mi'ut, a covering that extends over only a minor portion of the body, invalidates the tevilah whenever the person is makpid and wants the substance removed. And lechatchilah the standard is stricter still: one should go down to toivel entirely free of any covering at all.
A word of caution
The topic of chatzitzah is broad, and its details are fine ones. So let one point be set down plainly at the head of these mishnayos: whenever a chatzitzah question arises in practice, it must be placed before a rav who is a baki in this area, who commands these halachos in depth. Nobody should be paskening for himself, reading the mishnah and reasoning his way to a conclusion about whether some particular thing blocks the water or not. Such a she'eilah is the province of a moreh hora'ah, a talmid chacham who grasps the halacha and can render a genuine ruling in matters of this kind.
The Mishnah
The Mishnah opens: "Eilu chotzitzin ba'adam," these are the things that form a chatzitzah on a person. The list begins with "chutei tzemer, vechutei pishtan, veharetzu'os sheberoshei habanos," threads of wool, threads of linen, and leather straps that are on the heads of girls. These are the ties with which the hair was gathered and bound in bunches. Since they bind the hair, the water of the mikveh does not reach what is underneath them, and they are a chatzitzah.
The Mishnah then records a dissenting view: "Rabbi Yehudah omer, shel tzemer veshel se'ar einam chotzitzin," Rabbi Yehudah says that ties made of wool or of hair do not form a chatzitzah, "mipnei shehamayim ba'im bahen," because the water enters through them. Such materials do not hold a tight, sealed knot; the binding stays loose enough that the mikveh water passes through and reaches the hair beneath.
Let us restate the lesson of the mishnah as we have it. Tevilah demands direct contact between the water and every part of the body, including the hair, and even a thread or a strap tying up the hair can stand in the way. Rabbi Yehudah measures the material itself and rules that soft wool or hair lets the water through. And, as we said at the outset, we are learning the mishnah kifshuto, at face value, and not drawing practical conclusions from it.