Parah, Chapter 5, Mishnah 8. This chapter deals with the vessels used for the mei chatas, the water that is sanctified with the ashes of the parah adumah. Our Mishnah asks a question about the boundaries of a vessel: when do two hollows count as two separate containers, and when are they treated as one?
The Mishnah begins: "Shtei shokasos sheb'even achas" - two troughs hollowed out of a single stone - "kidesh achas meihen", he sanctified one of them, "hamayim shebashniya einam mekudashin", and the liquid sitting in the other hollow stays unsanctified. Kiddush of the water is accomplished by dropping in the eifer, the ashes of the parah adumah, and in our case the ashes went into just one of the two cavities. That both cavities were chiseled out of the very same block of rock does not turn them into a unit. Halachically each hollow stands on its own as a receptacle, so whatever was done on the one side leaves the water on the other side precisely as it was before.
Two circumstances, however, reverse that ruling. The Mishnah gives the first: "Haya nekuvah zu lazu", if this one was pierced through to that one, "keshefoferes hanod", with an opening as wide as the tube of a wineskin. And the second: "o shehayu hamayim", or if the water stood "tzafin al gabeihen", floating above the pair of them, "afilu k'klipas hashum", even by no more than the skin of a garlic clove. In either of these two cases, "v'kidesh es achas meihen", he sanctified the one, and then "hamayim shebashniya mekudashin": the water of the second one is sanctified along with it.
The first case is a connecting passage between the two hollows. If the opening between them is at least as wide as the tube of a wineskin, the standard measure for joining bodies of water, the two troughs are no longer separate. The second case is water that has risen high enough to cover the ridge of stone between them, even by the thinnest film, no thicker than the papery peel of a piece of garlic. That slender covering of water is enough to unite what is below it.
In either situation, when he places the ashes into the water of one trough, the water in the other becomes sanctified along with it, because the two are now regarded as a single receptacle holding one continuous body of water. The lesson of the Mishnah is that the sanctification follows the unity of the water, not the unity of the stone: one block of rock with two separate hollows gives us two vessels, while a connecting hole or an overflowing film of water gives us one.