Mikvaos, Chapter 6, Mishnah 4. Our mishnah continues the chapter's discussion of the ways in which mayim she'uvim, drawn water, does or does not affect a mikveh. The background rule is this: if three lugin of drawn water enter a gathering of rainwater before it has reached its full shiur of forty sa'im, the mikveh is invalidated. Here the Mishnah teaches a limitation on that rule, and the limitation turns on a single question: is the drawn water there as water?
Picture two cases. The first is a sponge that has soaked up three lugin of drawn water. The second is a pail, let us say an earthenware pail, whose walls have absorbed three lugin of drawn water, so that nothing is visible to the eye. In both cases the drawn water is present, but it is not standing there as water. It has been absorbed, and absorbed water is not considered water that is b'ein, actual and present in its own right.
Hence the language of the Mishnah: "hasfog v'hadli shehoyu bahen shloysho lugin mayim," a sponge and a pail that had in them three lugin of water. The water in question is mayim she'uvim, held either in the pores of the sponge or hidden inside the earthenware of the pail, where no eye can detect it. "V'naflu lamikveh," and they dropped down into the mikveh, sponge and all, and the ruling is "lo poslu," the mikveh suffers no disqualification.
The Mishnah supplies its own reason: "shelo omru ella shloysho lugin shenaflu." When the Chachamim established the halacha that three lugin of drawn water invalidate a mikveh, they framed the decree around three lugin that fall in, that is, three lugin of real, actual water entering the mikveh. Water that has been swallowed up inside a sponge or inside the walls of a vessel was never included in that decree. It arrives absorbed, not as falling water, and therefore the mikveh remains kosher.
The lesson to carry away is a precise one. The disqualification of three lugin is not measured by how much drawn water is somewhere in the vicinity, and not even by how much drawn water physically enters the mikveh. It is measured by drawn water that enters as water. Once it has been absorbed and is no longer b'ein, the three lugin lose their power to pasel.