Parah, Chapter 9, Mishnah 1. Once the ashes of the parah adumah have been placed into the water in the trough, the sanctified water is poured into a small vessel from which it will be sprinkled. The Mishnah calls that vessel a "tzlochis," a flask. Our mishnah takes up what happens when something else finds its way into that flask: ordinary water, dew, various liquids, or a substance that leaves a mark.
"Tzlochis shenafal l'sochah mayim kol shehen, Rabi Eliezer omer, yazeh shtei hazayos": a flask into which even the smallest amount of ordinary water fell, Rabi Eliezer says he should sprinkle two sprinklings. "Va'chachamim poslin": and the Sages declare it unfit.
Rabi Eliezer's ruling rests on an assumption about how liquids behave: two of them do not fuse into one uniform substance. The drop of plain water that entered keeps its own identity rather than spreading evenly through everything in the flask, so we do not regard the contents as hopelessly merged; in principle the two liquids remain distinguishable. And since the amount that fell in was tiny, it is inconceivable that both sprinklings would consist of nothing but that plain water: some measure of authentic sanctified water is bound to have gone along with it. Rabi Eliezer furthermore holds that a sprinkling has no minimum required quantity, so however little genuine sanctified water reaches its target, the sprinkling has done its work.
The Sages disagree and rule the flask unfit. In their view a sprinkling does require a minimum amount, and two sprinklings cannot be combined to make up that amount between them. Since each individual sprinkling may well contain some of the ordinary water, the sanctified water within it could amount to less than the required minimum, and so the whole thing is unfit. The halachah follows the Chachamim.
The next case in the Mishnah is dew: "Yarad l'sochah tal," dew came down into the flask. Rabi Eliezer offers a remedy, "yanichenah bachamah," the flask should be left standing where sunlight reaches it, "v'hatal oleh," and the dew will evaporate off, leaving the sanctified water as it was. "Va'chachamim poslin": the Sages rule the contents unfit, for they hold that dew which has already mingled with the water does not lift away again in this manner. In practice, the halachah follows the Chachamim here as well.
A further case: "Nafal l'sochah mashkin u'mei peiros." The term "mashkin" refers to the seven liquids: dew and water, wine and oil, blood, milk, and the honey of bees. "Mei peiros" refers to juices pressed out of produce, excluding grapes and olives, whose products belong to the list of seven. Should any such liquid make its way into the flask, the ruling is "ye'areh v'tzarich l'nagev": everything in the flask has lost its status and is poured off, and the vessel then has to be wiped out, since it may not receive sanctified water again as long as any moisture clings to it.
"Dyo, komos, v'kankantos, v'chol davar shehu roshem, ye'areh v'eino tzarich l'nagev": ink, gum, copperas (a dye produced from copper), or anything else that leaves a visible mark, he pours the water out and need not dry the flask. Here all the water must likewise be emptied, but drying is unnecessary, for a simple reason: if any residue of such a substance were left behind in the vessel, it would be plainly visible.