Parah, Chapter 7, Mishnah 12. Throughout this chapter the Mishnah works out one rule from every angle: a person engaged with the mei chatas, from the drawing of the water until its sanctification with the ashes of the parah adumah, may not interrupt himself with a melachah. Yet not every action is a disqualifying melachah. An act that serves the needs of the chatas, even a rather ordinary act, is part of the work itself and leaves the water fit. This Mishnah offers three illustrations, and in each one the outcome turns on that distinction.
Here is the Mishnah's wording, clause by clause. "Haporeitz al menas ligdor, kasher": one who opens a gap in a wall while planning to close it up again leaves the water valid. "Ve'im gadar, pasul": should he actually close it up, the water is ruined. "Ha'ochel al menas liktzos, kasher": a man who eats with the plan of spreading figs for drying leaves the water valid. "Ve'im katzah, pasul": once he in fact spreads them out, the water is ruined. And the third clause: "Hayah ochel", he was in the midst of a meal, "vehosir", and a portion was left over, "vezarak mah shebeyado", and he flung the remainder he was holding "letachas hate'einah", into the shade of the fig tree, "o letoch hamuktzeh", or into the spot set aside for drying figs, "bishvil shelo yovad", so that it would not be lost, "pasul", the water is disqualified.
Breaking through a fence to open a path
Picture a man transporting the drawn water toward the place where the ashes will be added, and a stone wall blocks his route. He opens a gap in it so that he can get through, intending all along to close it up once he has passed. Making the gap spoils nothing, because it was done on behalf of the chatas: without it the water could not reach its destination. His plan to repair the wall afterward does not turn the breach into an unrelated labor. Should he in fact close the wall up, however, that repair contributes nothing whatsoever to the water. It stands as a labor in its own right, and the water entrusted to him becomes pasul.
Eating in order to lay out figs
The second case involves a man who, after drawing the water, eats his fellow's figs, and he eats them on the understanding that he will later lay out that man's figs in the place where figs are spread to dry (splitting them and preparing them for drying, or pressing them into fig cakes) as payment for what he ate. Eating is not a melachah that harms the chatas; on the contrary, it gives him the strength to carry the water to its sanctification, so the eating serves the chatas and the water remains kasher.
Should he in fact spread those figs out to dry before the ashes had been added, the water is disqualified. Such labor bears no relation to the water at all, and there was no pressing reason to do it at that moment: nothing prevented him from attending to the figs once the sanctification was behind him. Because the task could easily have been postponed, performing it while the water was still in his charge turns it into an outside melachah.
Putting away what was left in his hand
The final case is the finest of the three. He had been eating, and part of what he had taken was still in his grasp. Not wanting it to be lost, he threw it into the shade of the fig tree, or into the spot where figs are set out to dry, so that it would be kept. Tossing food into a place of safekeeping is itself a melachah, an act of protecting produce, and it is performed for the sake of the food rather than for the chatas. The water is therefore disqualified.
The three cases together sharpen the same lesson. Breaking a fence and eating figs, which sound like ordinary weekday activity, are fit when they are the means of getting the water to its sanctification. Rebuilding the fence, spreading out figs, and even flinging a leftover piece of fruit into a safe place, which sound like nothing much at all, are disqualifying, because they serve something other than the mei chatas. Everything depends on whether the deed belongs to the service of the water.