Niddah, Chapter 5, Mishnah 5. This chapter has been moving through the stages of a person's life and showing how each age carries its own halachic consequences. Our Mishnah reaches a landmark age for a girl: three years and one day. From that point on, an act of relations with her counts as a real halachic act of bi'ah, with all the legal results that follow from it, and the Mishnah lists them one after the other.
"Bas shalosh shanim v'yom echad, miskadeshes b'bi'ah." Once a girl has passed her third birthday by a single day, she can be taken as a wife through relations. Kiddushin takes effect through more than one method, and relations is among them. Since her act now carries halachic weight, a man who betroths her in this way is genuinely married to her. The closing line of the Mishnah makes the contrast plain: any earlier, and what occurred is not classified as a sexual act at all, so nothing resembling kiddushin could emerge from it.
"V'im ba aleha yavam, kenanah." The same principle extends to yibum. Picture a case where she was married to a man who then died childless, so that she now falls to his brother for yibum. If that brother has relations with her, he has acquired her, and from that moment they are husband and wife in the full sense. Once she has this status of a married woman, the laws of adultery apply to her as well, since the age at which a woman's relations are halachically an act of bi'ah begins at three years and one day.
"U'mitamei es bo'alah l'tamei mishkav tachton k'elyon." If she is a niddah, the man who has relations with her receives the status of a boel niddah, one who has been with a niddah. That status carries a particular strength of tumah: he conveys tumah to an entire stack of coverings beneath him, so that the lowest cloth becomes tamei to the same degree as the uppermost one. This is precisely the tumah that defines a boel niddah, and a girl of this age is able to bring it about.
"Nisas l'kohen, ocheles b'terumah." The Mishnah stays with the same girl, past her third birthday by a day: should she become the wife of a kohen, terumah is permitted to her, since her marriage is halachically valid and it brings her into his household for this purpose.
"B'alah echad min hapesulin, pesalah min hakehunah." The reverse also holds. If she is still unmarried and one of the men whose relations render a woman unfit for the kehunah has relations with her, a non-Jew for example, he has disqualified her from marrying a kohen. Here too it is only because her relations count halachically that such a consequence can follow.
"Ba aleha echad min kol ha'arayos ha'amuros baTorah, mumasin al yadah v'hi peturah." Should any of the relatives whom the Torah forbids as arayos come to her, those men incur the death penalty because of her, for the prohibition has been transgressed in full. She herself goes free, since she has not yet arrived at the age when a person is held answerable for what she does.
"Pachos mikan, k'nosein etzba b'ayin." Everything listed until here applied from the age of three years and a day upward. Younger than that, the Mishnah offers its vivid image: the act is comparable to poking a finger into an eye. Some sort of physical contact has taken place, but halachah does not classify it as bi'ah, and consequently none of the outcomes above can be produced by it: no kiddushin, no kinyan through yibum, no invalidation for the kehunah, and no tumah of a boel niddah.
The Mishnah has thus drawn a single clear line. Three years and one day is the age from which a girl's relations become a legally recognized act, and the entire list of halachos in this Mishnah, in marriage, in yibum, in tumah, in terumah, in the kehunah and in the arayos, flows from that one determination.