Mikvaos, Chapter 4, Mishnah 3. Our chapter is occupied with rainwater that travels toward a mikveh through pipes and channels, and with the question of when that water picks up the disqualification of mayim she'uvim, drawn water. The principle behind the whole discussion is that rainwater must reach the mikveh without ever having rested in a vessel or in anything that functions as a vessel. In this mishnah we meet a pipe that has a small pocket carved into it, and the Tanna teaches us to distinguish between a hollow that was fashioned in order to hold something and a hollow that was never intended to hold anything at all.
Before entering the mishnah itself, it is worth noting that a large question hangs over this subject: when part of a pipe contains such a receiving pocket, does only the water that settles in the pocket become she'uvim, or does the entire flow passing through that pipe take on the status of drawn water? This is a broad and much debated matter among the Rishonim and the Acharonim, and it is worth seeing the Shach in Yoreh De'ah, hilchos mikvaos, se'if katan 77, together with the Nekudas HaKesef there, where the issue is taken up at length.
Carving a Pocket to Catch Pebbles
The mishnah opens: "Hachokeik batzinor kedei lekabeil tzroros", one who carves out a hollow inside a pipe in order to catch pebbles. The purpose of the carving is practical: small stones carried along by the water should settle into that pocket rather than clog the pipe or be washed straight through into the mikveh. Since the hollow was deliberately made to hold something, it stands a good chance of being treated as a beis kibul, a receptacle, which would render the water passing into it she'uvim.
The Tanna Kamma draws a distinction based on the material of the pipe. "B'shel eitz kolshehu": if the pipe is wooden, even the smallest carved pocket counts. Wood rots, and a tiny indentation will inevitably widen with time, so even a minimal hollow is already reckoned a receptacle, and water entering it becomes she'uvim. "U'v'shel cheres revi'is": if the pipe is earthenware, the hollow must be able to hold a revi'is before it is treated as a receptacle capable of turning the water into she'uvim.
Rebbi Yosi's Position
Rebbi Yosi disagrees. "Af b'shel cheres kolshehu": even in an earthenware pipe, any hollow at all, however small, constitutes a beis kibul. He clearly does not accept the Tanna Kamma's distinction between wood and earthenware.
Rebbi Yosi then explains where the measure of a revi'is actually belongs: "lo amru revi'is ela b'shivrei chlei cheres". The shiur of a revi'is was never stated with regard to defining a receptacle in the first place. It was stated with regard to broken pieces of earthenware vessels. A vessel whose capacity ranged from a log up to a se'ah, and which then broke, retains its status as a vessel so long as the surviving shard can still hold a revi'is. There the revi'is is the line between a broken piece that has lost the halachah of a kli and one that has not been nullified from toras kli. It has nothing to do with a hollow carved into a pipe.
Pebbles That Roll and Earth That Packs In
The mishnah continues with two cases in which the pocket has already collected material. "Hoyu tzroros misgalgelim": if the pebbles sitting in the hollow are loose and roll about as the water pushes past them, the pocket has not been sealed off. There is still an open receiving space there, so the water flowing into it is she'uvim, and "poslin es hamikveh", it disqualifies the mikveh.
But "yorad letocho afar venichbash": if earth washed down into the hollow and became tightly packed there, the receiving space has been nullified. The pipe no longer has a beis kibul along its length, and the halachah is kasher. From that point on the water traveling through the pipe is fine.
A Pipe That Bulges in the Middle
The mishnah closes with a very different sort of widening. "Silon shehu tzar mikan u'mikan verachav b'emtza": a pipe that is narrow at both ends and wide in the middle. The Rambam explains the purpose of such a design: the water backs up and builds pressure in the wide section, and then bursts out with force through the narrow opening. That bulge was never fashioned to hold water; it was fashioned to create pressure.
Therefore "eino posel", it does not disqualify. The widened section is not a beis kibul, the water passing through it is not considered she'uvim, and the mikveh remains kasher. The mishnah gives the reason itself: "mipnei shelo na'asa lekabeil", because it was not made in order to receive. The whole standard of this mishnah is intent and function. A pocket carved to catch pebbles is a receptacle; a bulge shaped to drive the water forward is simply part of the channel.