Yadayim, Chapter 1, Mishnah 4. The opening chapter of the maseches lays out the conditions of netilas yadayim: how much water is needed, what sort of vessel it must come from, and what can ruin the water so that it is no longer fit for washing the hands. One of those disqualifications is water with which work has already been done. Our mishnah presents a case where that rule looks like it should apply, and teaches us to look more closely at exactly which water was put to use.
The first half of the mishnah concerns water put to use in a bakery. The water, "hamayim," that the baker, "shehanachtom," is dipping into, "matbil bahen," the fine rolls he is preparing, "es hagluskaos," is "pesulin," disqualified for netilas yadayim. It was recruited for his baking, and once water has served a purpose of that kind it can no longer be poured over the hands.
But then the mishnah gives the other side of the coin: "V'chishehu madiach es yadav bahen, kesherin." If instead of dipping the rolls themselves, the baker dips his hands into the water in order to moisten them and then goes back to working the dough, the water in the vessel is still kosher for netilas yadayim.
The Bartenura's Distinction
What accounts for the difference? The Bartenura points to the location of the water. He rules that the supply remaining behind in the utensil holds its status: "kesherin hamayim shebekli l'netilah." And he gives the reason: no labor was performed, "shelo na'asis melacha," with the water still sitting in the utensil, "bamayim shebekli." Rather, the labor was done with a different portion, "ela bamayim," the water the baker had lifted out in his cupped palms, "shelakach b'chofnav."
In other words, when the baker plunges the rolls into the vessel, the vessel's water is itself the working material, and it is spoiled. When he merely wets his hands, whatever water he carries off in his hands is what goes into the dough; the supply left behind in the vessel was never used for anything. It remains exactly what it was before, plain water waiting to be poured over someone's hands.
The lesson is a precise one, and it is typical of Yadayim: the halachah does not ask in a general way whether water was "involved" in someone's work. It asks which water, and it follows that water to see what became of it.