Parah, Chapter 8, Mishnah 1. This chapter carries forward the subject of what keeps the mei chatas usable, the water into which the ashes of the parah adumah are mixed, and what spoils it. The person appointed over such water carries two obligations at once: his body must stay tahor, and his eyes must stay on the water. Should tumah reach him, or should he turn away to busy himself with some other melacha so that the watching stops, the water is lost. Our Mishnah takes up the case of a pair of watchmen rather than a single one.
Two watchmen, and one contracts tumah
The Mishnah opens: "Shnayim shehayu shomrim", two men were on watch, "es hashoket", over the trough of water. Then "nitma echad meihem", tumah reached one of the two. The ruling is "ksherim", the water stays valid, "mipnei shehein birshuso", since the water still stands in the keeping "shel sheini", of his companion. What ruins the water is its being abandoned without a fit watchman, and that never happened here: the second man's eyes were on it throughout.
The Mishnah continues: "Tahor venitma sheini", the first one underwent taharah and returned to his post, and then tumah reached the second one. Once more "ksherim", the water is valid, "mipnei shehein birshuso", because it now stands in the keeping "shel rishon", of the man who watched first. The two have merely traded places, and there was not a moment when the trough lacked a watchman who was tahor.
Then comes the failure: "Nitme'u shneihem ke'achas, psulim." Should tumah reach the pair of them simultaneously, the water is pasul. There is no longer anyone to rely upon, for both watchmen dropped out together and the water was left with nobody qualified over it.
The identical ruling regarding melacha
"Asah echad meihen melacha", one of the pair turned to some other job and so took his mind off the watching. Here too "ksherin", the water is valid, "mipnei shehein birshuso", for it remains in the keeping "shel sheini", of his partner, who is present and attentive. The second requirement follows exactly the reasoning of the first.
"Amad ve'asah hasheini melacha", the first man left off his job and resumed watching, whereupon the second man took up work of his own. Again "ksherin", the water is valid, "mipnei shehein birshuso", since it lies in the keeping "shel rishon", of the original watchman. So long as one of the two is truly attending to the water, the obligation of shemirah is being fulfilled.
And the closing clause: "Asu shneihem ke'echad, psulin." Where the pair of them occupied themselves with work at one and the same moment, the water is unfit, because in that moment no one at all was watching it.
One idea unites the whole Mishnah. The shemirah of the mei chatas does not have to be supplied by a single individual from start to finish; it simply may never be interrupted. Two men are free to pass the duty between themselves, whether tumah or melacha is what pulls one of them away, and the water remains fit. Only when the two of them fall out of the watch together does the continuity of shemirah snap, and with it the water is forfeited.