Machshirin, Chapter 3, Mishnah 7. Throughout this chapter we meet situations in which food, or a person, ends up wet although nobody set out to wet it, and the question each time is whether that water still qualifies as water the owner was happy to have. Only water of that sort is machshir, and the Mishnah's expression for it is "b'chi yutan," borrowed from the pasuk that speaks of water being placed upon food. Once the water has that status, whatever it came into contact with is huchshar l'kabel tumah, made fit to take on tumah.
The donkey drivers fording the river
"Hachamarim shehayu ovrim banahar v'naflu sakeihem lamayim." Men who transported goods by donkey had bought produce in one place and were bringing it to another to sell. While they were making their way across a river, the loaded sacks slid off the animals and landed in the water. The ruling: "im samchu, b'chi yutan." Should the wetting have given them satisfaction, the water counts as desired and the produce is now fit to receive tumah.
Rebbi Yehuda raises here the same challenge he raises again and again in this chapter: "ee efshar shelo yismach." No merchant alive fails to be happy when his goods are improved, and a test that is passed in every single case is not a test at all. Satisfaction on its own, then, cannot decide the halachah. What must be present is a maaseh, some action in which that satisfaction becomes visible. Rebbi Yehuda's standard is "ella im hafchu." The Rav explains that the drivers flipped the sacks around in the water so that the produce would be reached from every side. Such a move demonstrates that the water was welcome, and only in that case has the produce become huchshar l'kabel tumah.
Mud-covered feet in the river
A second case follows: "hayu raglav mele'os tit v'chein raglei behemto." A man's feet were thickly covered with mud, and his animal's feet were in the same condition. "Avar banahar": he passed through a river, with no intention of rinsing anything off, simply because that was the way his journey ran. The water, of its own accord, carried off the mud. The Mishnah teaches: "im samach, b'chi yutan." If the outcome pleased him, the water still clinging to him has the status of desired water and can serve to be machshir.
Compare this with the straightforward situation. A person who goes down into water in order to wash himself is left with some water on his body when he comes out, and that residue is counted as water he wanted, since he chose to go in and it is common knowledge that one emerges from water wet. Our traveler, by contrast, had no such purpose in mind when he entered the river. Nevertheless, the crossing did in fact rinse his feet and those of his animal, and so if the result is to his liking, the water remaining is b'chi yutan.
True to his position, Rebbi Yehuda responds: "ee efshar shelo yismach." Since nobody would fail to be pleased here, pleasure cannot serve as the measure. His condition is "ella im amad v'hediach." The Rav explains that he halted in the river and stayed put there for a while precisely so the mud would be rinsed away. Coming to a stop is the maaseh that lays bare his machshavah and establishes that the rinsing was something he sought. Then the water left on him has the power to be machshir, so that should it drip onto produce, that produce becomes huchshar l'kabel tumah.
Rebbi Yehuda's qualification
Rebbi Yehuda now attaches a significant limit: "v'adam u'veheimah temei'ah l'olam tamei." His demand for stopping and washing is limited to a kosher animal. Where a human being is concerned, and likewise a non-kosher animal, the water counts in every case, with no action needed to make the satisfaction evident.
Why? Because with a person and with a non-kosher animal, looks are something the owner cares about. Nobody has any hesitation about wanting mud and grime removed, so from the instant he is in the water it is plain beyond doubt that the cleaning is welcome, and nothing further need be done. A kosher animal stands elsewhere: its appearance is not necessarily of any concern, since such animals are generally raised with slaughter in view. There the satisfaction does not speak for itself, so an act is needed to display it. With a person or a non-kosher animal, however, even Rebbi Yehuda holds the water desired without any maaseh whatsoever to give expression to the simchah. On that note our Mishnah closes.