Parah, Chapter 5, Mishnah 9. This chapter deals with the vessels in which the water of the Parah Adumah is prepared, and our mishnah asks a question about the identity of a vessel: when a body of water sits partly in one receptacle and partly in another, do we have one container holding water fit for the kiddush, or two containers whose shared water belongs to neither?
The Mishnah presents three cases. "Shtei avanim shekifan zo lezo va'asan shoket": two stones that a person placed one against the other so that the hollow between them would serve as a trough. "Vechen shtei areivos": and likewise two kneading tubs stood side by side, so that the water spans from one into the other. "Vechen hashoket shenechlekah": and likewise a single trough that cracked and split into two pieces which still stand together.
All three cases receive one and the same verdict: "hamayim shebeineihem einam mekudashin." Whatever water fills the space they share is unfit to have the ashes placed into it. To the eye it may well look like a single continuous pool, yet in halachah no vessel exists here at all. All that has happened is that two independent items were set beside each other, and mere closeness does not turn two objects into one container.
Now the Mishnah supplies the remedy. "Asan besid o begipsis": he fixed the two pieces to one another with lime or with plaster, "vehen yecholos lehinatel ke'achas," with the result that the pair can be picked up as a single piece. In such a case, teaches the Mishnah, "hamayim shebeineihem mekudashin": whatever water stands in the shared space may indeed receive the ashes.
The test, then, is whether the two pieces have truly become one. Once they are cemented with lime or gypsum, lifting one raises the other along with it; they move together as a single body. That is the mark of one vessel, and water held in it is fit to be sanctified with the ashes. Without that bonding, however tightly the two pieces are pressed against each other, they remain two, and the water resting between them cannot be sanctified.