Machshirin, Chapter 4, Mishnah 2. The masechta as a whole is built around one idea: detached water only makes food capable of receiving tumah when it came onto that food with a person's approval, beratzon. Our chapter keeps testing that principle against ordinary situations, and here the Mishnah asks about the most ordinary situation of all: a person walking outside when it begins to rain.
The case begins with the words "Mi sheyardu alav geshamim": a man over whom rain has come down, so that drops of water are now resting on his body and his clothing. The Mishnah then pushes the case to its limit with the words "afilu av hatumah": this holds true even where the man carries the gravest level of personal tumah there is, an av hatumah, such as someone who contracted tumah from a corpse. Nevertheless the ruling is "einan bekiyutan": those drops are not reckoned as water of any consequence, and they cannot function as a machshir for anything they touch.
To appreciate why that is striking, we need to recall a general rule. Ordinarily, water that is itself tamei acts as a machshir whether or not the owner wanted it there. The requirement of ratzon governs tahor water; once the water is tamei, it prepares whatever it touches to receive tumah in any case. So here, one would expect the rain that landed on an av hatumah, and thereby became tamei from him, to be a machshir without further discussion.
The Mishnah teaches otherwise. The decisive point is how the water arrived in the first place. Nobody wants to be soaked by a rainstorm; the water came down on him against his wishes entirely. Since its arrival was never beratzon, it does not enter the category of water that can prepare anything for tumah, even though contact with this man has rendered it tamei.
"Im nier, bekiyutan." If, however, he shook himself off, deliberately moving his body to get the water off him, the water now is bekiyutan. By taking action regarding the water he has given it chashivus, a measure of importance and relevance to himself, and that turns it into water that matters. From this point on, if that water touches other items, it renders them able to receive tumah.
A second scenario rounds out the Mishnah. "Amad tachas hatzinor": a man took up a position underneath a spout through which water was pouring, and he did so "lehakeir o lidoach," either in order to cool his body down or in order to wash himself off. Regarding a man in a state of tumah the ruling is "betamei, tamei": the stream picks up tumah from him, and it does so at the very instant that it also becomes water that qualifies as a machshir, bevas achas. One and the same water is rendered tamei by his body and is simultaneously fit to prepare other objects to receive tumah upon contact.
"Uvetahor, bekiyutan." And if the man is tahor, the water is still bekiyutan. The difference between this case and the opening case of the Mishnah lies in the way the water reached him. Rain descending from the sky comes upon a person whether he likes it or not. Here he chose to stand beneath a spout so that the water pouring out of it would run over him, and even though what flows through that pipe is nothing other than rainwater, his own decision to stand there makes it water that arrived beratzon. It therefore serves as a machshir when he is tahor, and when he is already tamei it becomes tamei and a machshir together.
The lesson of the Mishnah, then, is the difference between water that happens to a person and water that a person chooses. Getting rained on is something we endure; shaking ourselves off, or stepping under a spout to wash or to cool down, is something we do. In the laws of hechsher, that distinction is everything.