Mikvaos, Chapter 2, Mishnah 7. To appreciate this mishnah we need to recall the basic rule that governs the entire tractate: a kosher mikveh is formed from mei geshamim, rainwater that gathers on its own. The water may not be mayim she'uvim, drawn water, meaning water that was collected in a receptacle or that reached the mikveh through human handling that carried it along.
Our mishnah presents a very ordinary scene. A man is hamaniach kankanim berosh hagag, he sets jugs out on his roof, and his purpose is lenagvan, to dry them. Presumably they were still somewhat damp inside, and the sun beating down on the roof would do the job nicely. Then something he did not plan for happened: venismalu mayim, rain came down and the jugs filled with water.
The question is straightforward. This water landed inside vessels. Has it thereby taken on the status of mayim she'uvim and become invalid for a mikveh, or may he still make use of it?
The Ruling of Rebbi Eliezer
Rebbi Eliezer omer: everything depends on the circumstances. Im oinas hageshamim hu, if this occurred during the rainy season, and im yesh k'mat mayim babor, and there is already at least a small amount of rainwater sitting in his pit, then yishaver, he may break the jugs. Once they are broken, the water spills out onto the roof and runs down on its own into the mikveh below. What he may not do is lift the jugs and pour their contents in, because the moment he lifts and pours, the water becomes she'uvim.
Ve'im lav, but if the pit is empty and the entire mikveh would have to be formed from the water in these jugs alone, then lo yishaver, breaking them is of no help to him. Rebbi Eliezer permits this water only as an addition to rainwater that is already in the pit. Since the water was, after all, received inside a vessel, it cannot serve as the mikveh on its own.
The Ruling of Rebbi Yehoshua
Rebbi Yehoshua omer: what settles the matter is the owner's purpose at the moment he set the jugs down, and that purpose had no connection at all to collecting water. He wanted them dried out, nothing more. Consequently the law is the same in either situation, bein kach u'vein kach, rainy season or not, water already waiting in the pit or a pit standing empty: yishaver, he is permitted to smash them, o yichpeh, or he may turn them upside down and let the water spill where it falls.
Aval lo ye'areh, what remains forbidden is picking up a jug and emptying it into the mikveh. Doing that brings the water into tfisas yedei adam, the grasp of a person's hands carrying it from place to place, and with that the water becomes she'uvim.
So this mishnah teaches us how much weight the halachah places on the difference between water that a person actively carries and water that he merely allows to run its own course, and how much weight Rebbi Yehoshua places on the absence of any intention to collect water in the first place.