Parah, Chapter 7, Mishnah 4. This chapter deals with the melachah that spoils the water prepared for the parah adumah. Between the drawing of the water and its sanctification, when the ashes are placed into it, the person occupied with it may not turn aside to other labor. Two rules govern everything in our Mishnah: melachah damages only the water of the one who does it, and once the water has already been sanctified, later melachah no longer harms it. Our Mishnah applies these rules to two men doing favors for one another, and the results differ according to which favor comes first.
The four cases
Four exchanges are recorded here. "Kadesh li", add the ashes to my water, "va'akadesh lach", and I will do the same for yours: "harishon kasher", only the earlier of the pair holds up. "Malleh li", draw water on my behalf, "va'amalleh lach", and I will draw on yours: "ha'acharon kasher", and here it is the later one that survives. "Kadesh li va'amalleh lach", put the ashes in mine while I do the drawing for you: "shneihem k'sheirin", and nothing is lost. "Malleh li va'akadesh lach", do my drawing while I put the ashes in yours: "shneihem psulin", and both are ruined.
Sanctify mine and I will sanctify yours
Picture two men, each of whom has drawn water for his own sanctification, and one proposes an exchange of services. The first sanctification is valid: at the moment his fellow places the ashes into his water, that fellow has not yet done any melachah, and once the water has been sanctified, whatever melachah follows can no longer affect it. The second sanctification, however, is invalid. The man whose water was sanctified first now performs the melachah of sanctifying his colleague's water, and he does so while his colleague's water is still waiting to be sanctified. That colleague's water was therefore spoiled by its own owner's labor before its sanctification.
Draw for me and I will draw for you
Here the order of validity is reversed and the last one is fit. The water drawn first is disqualified, because its owner then went and drew water for his colleague, a melachah performed in the interval between the drawing of his own water and its sanctification. The water drawn second suffers no such flaw, since its owner did nothing in the gap between its drawing and its sanctification.
Sanctify mine and I will draw for you
In this arrangement both are valid. The sanctification is good because the owner did his melachah, drawing for his colleague, only afterwards, and melachah after sanctification is harmless. And the water drawn after that sanctification is also good, because its owner, having already finished his part, does no melachah between its drawing and its sanctification.
You draw mine, I sanctify yours
Here everything collapses, for the opposite reason. The water drawn first is invalid because its owner occupied himself with sanctifying his colleague's water in the interval between his own water's drawing and its sanctification: a melachah in precisely the wrong moment. And the water that had been drawn earlier is invalid as well, since its owner performed the melachah of drawing water for his colleague while his own water still awaited sanctification.
The whole Mishnah turns on a single sensitive interval. From the moment a man's water is drawn until the ashes are placed into it, he must remain free of other labor, and an act of kindness done for a friend in that window is no less a melachah than any other work.