Yadayim, Chapter 2, Mishnah 1. The opening chapter of the tractate established the framework of netilas yadayim: a revi'is of water, poured from a vessel, water that has not been spoiled by prior use. Our mishnah opens the second chapter by asking how far a single pouring can be stretched, and then by asking what becomes of the water itself once it has done its job.
One hand or two
"Notel l'yado achas mishtifah achas, yado tehorah." If a person pours water over one hand in a single pouring, that hand is purified.
The Bartenura clarifies the case. A single pouring suffices for one hand even though the vessel does not hold a full revi'is, because the water being used comes from shirei taharah, the remainder of water that already served validly for a washing, as was taught at the very opening of the tractate.
"Lishtei yadav mishtifah achas, Rabbi Meir metamei ad sheyitol meirevi'is." If he tries to cover both hands with that one pouring, Rabbi Meir holds his hands remain impure until he washes from a full revi'is. Here too the Bartenura explains: even though this water comes from shirei taharah, it will not do for two hands unless he pours twice, the first waters reaching up to the wrist and the second waters likewise reaching up to the wrist.
The loaf that falls into the water
"Nafal kikar shel terumah, tahor. Rabbi Yose metamei." A loaf of terumah falls into the water that has already been poured over his hands. The first opinion rules that the loaf stays pure; Rabbi Yose rules that it becomes impure.
The Bartenura sets the scene: we are speaking of a person who washed his first waters from a full revi'is, and a loaf of terumah then fell into those waters. The same question arises if he touched the loaf with hands still wet, whether before he dried them from the first waters or before the second waters were poured over them.
In all these cases the first opinion holds that the loaf remains tahor, and the reasoning is straightforward: if this water has the power to purify his hands, it certainly has the power to keep itself pure.
Rabbi Yose sees it otherwise. He agrees that the water accomplishes its purpose and purifies the hands, but he holds that in the very act of doing so the water itself becomes impure. A loaf of terumah that meets such water is therefore rendered tamei.
The mishnah thus leaves us with a fine distinction to carry forward: water can do its work faithfully and still be changed by the work it has done.