Yadayim, Chapter 2, Mishnah 2. This chapter examines the details of netilas yadayim, and especially the interplay of the two pourings: the first water removes the tumah from the hands and becomes tamei in the process, while the second water is tahor and purifies the first water still resting on the hands. Our mishnah adds a new element: the boundary line on the arm within which all of this tumah and taharah operates, and then several cases of hands washed one at a time or washed together with others.
The mishnah begins by drawing a line on the arm. Hands, it teaches, "Hayadayim mitam'os umitaharos," both contract tumah and are made tahor, but only "ad haperik," as far as the wrist joint. Past that line the whole subject of tamei and tahor hands has nothing to say. The mishnah then asks "keitzad," in what way, and answers with two cases that look nearly the same yet end in opposite rulings.
Case one: "Notal es harishonim" (he applied the first water) "ad haperik" (within the wrist), and afterward "es hasheinim chutz laperik" (the second water higher up, past the wrist), "vechazru layad" (and it ran back down onto the hand). The ruling is "tahorah," the hand is tahor. Case two: this time both pourings, first and second alike, landed "chutz laperik," beyond the wrist, and from there flowed down to the hand. Now the ruling is "temeiah," the hand is tamei.
The Bartenura brings out what separates the two rulings. In the opening case the first water never traveled past the wrist joint, "dekeivan derishonim lo yatz'u" (since the first water did not go out) "chutz laperik" (beyond the joint). Consequently "lo nitme'u hasheinim" (the second water picked up no tumah) "bi'yetziasan chutz laperik" (as it passed through the region above the wrist), since there is no tumah of the hands there for it to absorb. It lands on the hand fully tahor and purifies the first water sitting there.
In the second case, though, everything begins in the wrong place. "Kesheyatze'u harishonim vesheinim" (when both the first and the second water went out) "chutz laperik vechazru layad" (past the joint and then flowed down to the hand), the hand is tamei, "lefi shenitme'u hasheinim" (because the second water was rendered tamei) "berishonim shechutz laperik" (by the first water lying above the wrist). Nor can the second water repair the situation, "she'ein hasheinim metaherim" (the second water does not purify) "harishonim hayotzim chutz laperik" (first water positioned beyond the joint); "ela ad haperik bilvad" (only within the wrist) "hein metaherin" (does it purify). The entire system of tumah and taharah here operates on the hand alone, from the joint downward.
Next the mishnah discusses washing the hands one after the other rather than together. A person directed the first water onto a single hand, "Notal es harishonim leyado achas," then had second thoughts, "venimlach," and gave the second water to the pair of hands at once, "es hasheinim lishtei yadav." Both hands end up tamei. The Bartenura traces why: the first water still resting on the hand that was never washed, "demayim she'al gav zu," conveys tumah, "metamin," to the water lying on the other one, "mayim she'al gabei chavartah." The second pouring has therefore achieved nothing.
Invert the order and the result inverts with it. Here the first water was given to the pair together, "Notal es harishonim" "lishtei yadav," and then he thought again, "venimlach," and applied the second water, "venotal es hasheinim," to a single hand, "leyado achas." The ruling is "yado tahorah": that hand is tahor. The second water did on it exactly what second water is meant to do to the water lying there, and the hand beneath emerges clean.
A further case: "Notal leyado achas" (he washed one hand only) "veshifshefah bachavartah" (and then wiped it dry against its fellow) - "temeiah," it is tamei. The Bartenura lays out the sequence step by step. The hand he wiped against was one "shelo naslah," a hand no water had touched. Therefore "nitme'u hamayim she'aleha" (the water on the washed hand became tamei) "machmas chavartah delo naslah" (through contact with the unwashed hand at its side). And once tamei, "vechozrin umitamin" (that water turns back and conveys tumah) "es hayad shenatal" (to the very hand that had just been washed).
And if he wipes his wet hand elsewhere? "Berosho o vakosel, tahorah." Should he dry it on his hair or press it to a wall, the hand keeps its taharah. A head and a wall are not unwashed hands; they have no tumah to give the water, and so nothing travels back onto the hand.
The closing clause is thoroughly practical. "Notlin arba'ah vachamishah": one pouring may serve four or five people at once, whether they position themselves "zeh betzad zeh," side by side, or "zeh al gabei zeh," with one set of hands held above another. A single stream covers them all, "uvilvad sheyarpou," as long as the hands are held slackly and kept apart, "sheyavo bahem hamayim," so that the water genuinely arrives on each one. The Bartenura points to the danger being guarded against: "shelo tiyei sham chatzitzah," no interposing barrier is permitted; hands pressed tightly against each other form a screen, and the water never truly touches what it must touch.