Parah, Chapter 5, Mishnah 3. This chapter is occupied with the vessels used in preparing and handling the waters of the chatas, the water into which the ashes of the parah adumah are placed. Our mishnah takes up a homely, ordinary utensil: a gourd shell that was dried out and hollowed, which people used as a container for drawing water. The question is what happens once such a shell has soaked up water that is not fit for sanctification.
Read the mishnah's words piece by piece. "Keruyah shetiviluha", a gourd shell that was given an immersion, "bemayim she'einan re'uyin lekadesh", in water that does not qualify for use in sanctifying. The ruling follows: "mekadshin bah ad shetitamei", sanctification may be performed in that shell right up until the moment it contracts tumah. And then the reversal: "nitmah, ein mekadshin bah", once tumah has reached it, it is no longer used for sanctifying.
Let us unpack the situation. The water in which the shell was immersed was not spring water, the only kind of water fit for the chatas; it was ordinary water such as that of a mikveh. Note carefully that this water was not impure in any way. A freshly made shell has had no opportunity to contract any tumah at all, so strictly speaking no immersion was needed. Still, as an added measure of care taken specifically for the chatas, the shell was immersed. After that immersion one may pour spring water into it and perform the sanctification.
Why is there no objection here, seeing that moisture drawn into the walls of the shell could leach back out into the spring water poured inside? Because whatever trickles back is a negligible amount of plain, pure water, lost inside a far greater quantity of valid water and reckoned as nothing at all. A concern that slight cannot generate a prohibition. There is even a view that once the shell has been left to dry out, it may be used for sanctifying with no reservation whatsoever.
The picture changes if the shell became tamei and was then purified by immersion. Now one may not sanctify the water of the parah adumah in it, for here we do worry: a single drop released from within the walls would carry tumah and render the sanctified water impure.
Rebbi Yehoshua refuses to accept the distinction the first tanna has drawn. His words: "Im mekadesh hu bah bitchilah", if the shell may serve for sanctification when we start out with it, then "af b'sof yekadesh bah", it should serve for sanctification at the later stage too, that is, even after it contracted tumah and was purified in a mikveh. And the reverse: "im eino mekadesh bah b'sof", if it is unfit for sanctifying at that later stage, "af lo bitchilah", then it was equally unfit at the outset, given the unsuitable liquid it releases. Rebbi Yehoshua's own leaning is toward the stricter side: even a shell that never contracted tumah at all should not be used for sanctifying, precisely because of the moisture stored in its walls that will eventually work its way into the spring water.
The mishnah's last clause: "Bein kach u'vein kach", in whichever situation we find ourselves, before the shell became tamei or afterward, "lo yosif l'socha mayim mekudashim", water that has already been sanctified is not to be collected into it. The measure applied here is tighter, since sanctified water is ruined by even one drop of unsanctified water emerging from the walls of the shell, and that stringency holds in every case. Most commentators decline to read this closing line as a continuation of Rebbi Yehoshua's position; they assign it instead to the first tanna.
The Rambam, writing in his laws of the parah adumah, states the halachah accordingly: a gourd shell immersed in water suitable for sanctifying may be used for the sanctification, but the water of the chatas is not collected into it, since such a shell draws liquid into itself, which is just what our mishnah has taught.
The lesson of the mishnah is the sensitivity of the chatas waters to the smallest trace of anything foreign. A vessel that looks perfectly clean and is perfectly clean may still be holding, invisibly inside its walls, a drop that the Torah's law of the parah adumah refuses to overlook.