Machshirin, Chapter 4, Mishnah 3. This masechta revolves around a single principle: water that has become detached only renders food capable of receiving tumah when it came there with a person's consent, beratzon. Our chapter probes that principle through everyday scenes, and this Mishnah presents a case where the very same act, placing a dish outside in the rain, can go either way depending on what the person had in mind.
The first half of the Mishnah: "Hakofes ke'arah al hakoseil": one inverts a bowl and rests it upon a wall, "bishvil shetidoach," his goal being that the rain should give it a rinsing. Imagine a dirty dish that its owner leaves out in a downpour exactly because he is counting on the rainfall to clean it. In such a case there is no guesswork about what he had in mind: he welcomes this water onto the dish, so the rain that drips off it is water that arrived beratzon. Such water is "bekiyutan," it has full standing, and it serves as a machshir, so that anything it comes into contact with becomes susceptible to tumah.
The second half: "Im bishvil shelo yilkeh" the wall, that is, if he propped the overturned bowl there as a shield so that the rain would not ruin the wall, then the verdict is "einan bekiyutan." Notice that the physical act has not changed a bit. He himself carried the vessel outdoors and positioned it where a soaking was guaranteed. But the entire point of what he did was to block the rain rather than to attract it. The bowl here is serving as a roof of sorts, a device for keeping the downpour off the wall, and the fact that the bowl gets drenched is a result he had no interest in whatsoever. Moisture that lands on something while its owner is busy trying to fend that moisture off never qualifies as water that came beratzon, and it leaves everything it touches unprepared for tumah.
Two identical scenes, then, a dish resting upside down on a wall in the rain, and the halacha turns entirely on the thought behind the act. When the rain is being put to work, the water counts. When the vessel is being used to fend the rain off, the water does not.