Mikvaos, Chapter 2, Mishnah 9. Our chapter has been occupied with the laws of mayim she'uvim, drawn water, and our mishnah carries that discussion one step further: when does rainwater that has entered a vessel take on the status of she'uvim, and when does it not?
The principle behind this mishnah was established earlier in the chapter. Water that collects in a vessel shelo l'da'as, without anyone intending to catch it there, is generally not treated as drawn water, and the discussion then turned to how one makes sure that such water does not end up acquiring that status later on.
The case of the mishnah
The mishnah presents a person who owns earthenware jugs and sets them down inside the mikveh: "Hamsader kankanin b'soch habor", he arranges jugs inside the pit. Why would anyone do this? The purpose was to soak the outer walls of the jugs so that they become saturated with water. Once the walls have drunk their fill, they will no longer draw off the liquid that he intends to store inside them.
What then happened? "Venisml'u mayim", they filled up with water. The jugs sat in the mikveh long enough that the water worked its way through the walls and reached the inside of the vessels. Earthenware is boleya, extremely absorbent; cheres is porous by nature, and water seeps from the outer surface straight through to the inner cavity.
The halacha
Now comes the ruling. The mishnah says "Af al pi shebalah habor es meimav". The picture is of a mikveh that has, so to speak, drunk itself dry: the water that once stood in the pit has passed through the walls of the jugs and is now held inside them. Nevertheless the mishnah concludes "harei zeh yishbor", that the owner may smash the jugs. He need not first haul them out of the pit and empty them elsewhere; he shatters them exactly where they sit, and the water that spills out of the broken vessels flows back into the mikveh with its status intact, not ruled she'uvim.
This ruling lends support to the position of Rebbi Yehoshua. Where water entered a vessel shelo l'da'as, with no intention of collecting it there, the halacha is that it never becomes drawn water at all, and it may therefore rejoin the mikveh and count toward it.