Mikvaos, Chapter 4, Mishnah 5. Our chapter has been occupied with the laws of mayim she'uvim, drawn water, and in particular with the question of when water that has gathered in some sort of container is treated as drawn. This mishnah presents two cases that look almost identical to the eye, and yet stand on opposite sides of the line.
A hollow in the rock
The mishnah opens: "Hashoches sheba'sela", a trough or hollow found in a boulder. Picture a rock with a depression carved into it, a cavity in which water can pool and stand. The critical point is that this rock is part of the ground, joined to the earth, and the hollow was formed in it while it was already attached. Such a cavity never acquired the halachic identity of a keli, a vessel, and the mishnah spells out four consequences of that fact.
"Ein memal'in mimenu": water may not be filled from it for use with the Parah Adumah. The Torah insists that the spring water used together with the parah's ashes be gathered into a vessel, and this cavity in the boulder has no such status.
"V'ein mekadshin bo": one may not place the ashes of the parah into it in order to mix them with the water, since the mixing too must take place in a keli.
"V'ein mazin mimenu": one may not sprinkle from it. The kohen who sprinkles the mixture of water and ashes upon a tamei person must do so from a vessel, and again, a hole in a rock does not qualify.
The fourth consequence touches the laws of tumas ohel. When a meis lies in a roofed area, the tumah spreads throughout that space. A kli cheres, an earthenware vessel, standing in that room protects its contents only if it is sealed with a tzamid patil, a tight-fitting closure. But that requirement applies specifically to a keli. Our hollow in the rock, since it is not a keli, needs no tzamid patil at all: a simple cover placed over it serves as a barrier and saves whatever lies inside, preventing the tumah from reaching the contents.
And finally, "v'eino posel es hamikvah". Should rainwater fall into this cavity and flow onward into a mikvah, the mikvah is not disqualified. The water was never held in a vessel, so it never became she'uvim.
When the vessel came first
Now the mishnah reverses the order of events. "Haysah keli v'chibrah b'sid": the stone was originally a loose rock, a cavity was hollowed out in it so that it became a genuine vessel, and only afterward was it fastened down to the ground with cement. Here every ruling flips. "Memal'in bah, u'mekadshin bah, u'mazin mimenah": all three services of the Parah Adumah may be performed with it, for it carries the status of a keli. In the tent of a meis it now "tzerichah tzamid patil", it requires a sealed closure in order to protect its contents, exactly like any other earthenware vessel. And "u'poseles es hamikvah": rainwater that collects in it becomes she'uvim, and it disqualifies the mikvah.
Undoing the status of a vessel
The mishnah goes on to teach how that vessel status can be undone. "Nikvah milematah o min hatzad": a hole develops in it, either in its floor or along its wall, with the result that not the slightest amount of water can remain standing inside it. Once the stone cannot hold water at all, it forfeits its standing as a keli, and the mishnah rules "kesheirah": it no longer has any effect on water that runs through it, and that water does not turn into she'uvim.
What size hole accomplishes this? Throughout Mikvaos the standard is "k'shefoferes hanod". To empty a wineskin or a barrel, a tube was set into it so that the liquid could run out, and when the flow was to be stopped the opening was blocked with a plug. The width of such an opening was reckoned at two fingerbreadths. Since the measure allows a finger to be turned freely inside the hole rather than merely fitting into it snugly, the practical figure comes out to something in the range of three inches.
The trough in Yerushalayim
Rebbi Yehudah ben Beseira takes issue with the ruling of the Tanna Kamma, and he supports his position with an actual case. "Ma'aseh b'shokes Yeihu shehaysah b'Yerushalayim": the trough of Yeihu that stood in Yerushalayim was a hollowed stone that had been made into a vessel before it was attached to the ground. "V'haysah nekuvah k'shefoferes hanod", a hole the measure of the shefoferes hanod had been opened in it, and even so, every immersion required for matters of taharah in Yerushalayim was performed there. That is to say, the water flowing through this trough was treated as fit rather than she'uvim, and the mikvah it supplied was used with confidence.
The mishnah records the response: "v'shalchu Beis Shammai u'parsuha". Beis Shammai sent word that a hole of that size is not enough. To strip such a trough of its status as a keli, so that the water passing through it should not be rendered she'uvim, a far larger breach is needed. "SheBeis Shammai omrim, ad sheyipaseis rubah": only when the greater part of it is broken away does it cease to be a vessel, and only then will water entering it remain fit.
The lesson of the mishnah is the sharpness of the dividing line. Two stone hollows can be indistinguishable in appearance, yet the one carved while joined to the earth never becomes a vessel at all, while the one carved before it was cemented down carries the full weight of a keli, in the service of the Parah Adumah, in the laws of tumas ohel, and in the disqualification of a mikvah.