Parah, Chapter 7, Mishnah 1. This chapter continues to deal with the melachos, the acts of labor, which render the water of the chatas unfit. The rule under discussion applies from the moment the water is drawn until the ashes of the parah are cast into it, and our Mishnah explores what happens when a person draws water into several jars and then changes his mind about how many kidushin, how many separate acts of placing in the ashes, he intends to perform.
The Mishnah's first case involves a group. "Chamishah shemil'u chamesh chaviyos": five men each drew water, five jars in all. Their plan was that every jar would receive its own placing of the ashes, five distinct acts of kidush, and afterwards they consulted one another and settled instead on a single kidush covering all five. Or picture the situation in the opposite direction: at the time of the drawing they had one joint kidush in mind, and later they resolved to sanctify each jar by itself. Whichever way the change of plan ran, the Mishnah's verdict is the same, "harei kulan kesharim": not one of the five jars is spoiled.
The Mishnah now presents the identical set of facts where a single man does all the drawing: "Yachid shemilei chamesh chavios", one individual who filled the five jars. He drew them meaning to sanctify each one separately, five acts of kidush, and afterwards he decided that a single kidush would serve for all of them. In that case, the Mishnah teaches, "ein kasher ela acharon": of the five jars, only the one filled last may still be used.
Why should this be? Since he intended each jar to be sanctified on its own, each filling stands as an independent act. When he went and filled the second jar, that filling constituted a melachah which came between the drawing of the first jar and its kidush. There is a hefsek, an interruption, and the water of the first jar is therefore disqualified. The same applies down the line: filling the third jar renders the second unfit, filling the fourth renders the third unfit, and filling the fifth renders the fourth unfit. Each successive filling invalidates the one before it.
The fifth jar, however, has nothing following it. No further melachah was performed after it was filled, so as long as its water was not mingled with the water of the others, it remains fit. Hence the Mishnah's ruling: ein kasher ela acharon, only the last one is good.
Next the Mishnah inverts the sequence. This time the man drew the five jars with a single joint kidush in mind, and only afterwards decided to sanctify each jar on its own, five acts of kidush. Here the ruling is that only "zeh shekidesh rishon", the jar he sanctified at the very start, is fit. So long as he regarded the five fillings as one continuous undertaking, no filling interposes itself between another jar's drawing and its kidush. But the moment he begins consecrating them one at a time, that opening kidush itself has the status of a melachah, and it stands between the drawing of the four jars still waiting and the kidush each of them has yet to receive. Such an act is a hefsek, and those four are unfit.
The Mishnah closes with two clauses in which a second person is brought into the picture. If the man who drew the water tells his fellow "kadesh lach es eilu", take these and consecrate them for your own use, the ruling is "ein kasher ela rishon", only the jar sanctified at the beginning is fit. But if he tells him "kadesh li es eilu", attend to the kidush of these jars on my behalf, then "harei kulam kesharim", all five may be used.
In the first case, the man who drew the water handed over the jars and told his fellow to sanctify them for himself. By handing them over he made the jars the property of the other man, and that man then went ahead and performed five separate kidushin. Since the water now belongs to the one doing the sanctifying, his position is exactly like the earlier case of a person who drew with one kidush in mind and then reconsidered and made five: the first kidush is a melachah which interrupts and disqualifies the others. Only the first jar is fit.
In the second case, the one who drew the water did not transfer the jars at all. He simply asked his fellow to perform the kidush on his behalf, on his own five jars, which he had filled with a single kidush in mind. Even though the fellow went and performed five separate kidushin, all the jars remain fit. The owner who drew the water did no melachah himself, and the man who performed the kidush cannot invalidate through his melachah water that does not belong to him. Therefore nothing here creates a hefsek, and all five jars are good.
The thread running through the entire Mishnah is the same: the water of the chatas is guarded from the moment it is drawn, and any intervening act of labor between the drawing and the kidush severs the two and renders the water unfit. Whether a given filling or a given kidush counts as such an interruption depends entirely on how the person intended to sanctify the water, and on whose water it is.