Machshirin, Chapter 3, Mishnah 6. Our chapter continues to work through cases in which produce becomes wet and we must decide whether that wetting counts as having been wanted by the owner. Only water that comes onto food with the owner's will can be machshir, that is, can prepare the food to be able to receive tumah. The Mishnah's standing phrase for water that qualifies is "b'chi yutan," the language of the pasuk that speaks of water that is put upon the food, and once the water is b'chi yutan, the produce is huchshar l'kabel tumah.
The case here: "hazeisim nesunim bagag, v'yardu aleihen geshamim." A person had spread out his olives on the roof, and rain came down on them. Rain is not something he brought, and he certainly did not pour it on. But olives that get wet are sometimes improved by it. So the Mishnah rules: "im samach, b'chi yutan." If he was pleased that the rain fell on them, the water counts as water he wanted, and the olives are now prepared to receive tumah.
Rebbi Yehuda disagrees, exactly as he does elsewhere in this chapter: "ee efshar shelo yismach." It is simply not possible that a person will not be glad about something that benefits his olives. Once gladness is guaranteed, making gladness the test empties the condition of any meaning; in Rebbi Yehuda's view the Mishnah would be laying down a condition that can never fail to be met. Therefore inner pleasure cannot be the measuring rod. What is needed is a maaseh, an action on the owner's part that puts his satisfaction on display.
And so Rebbi Yehuda gives us the actions that would qualify: "ella im pakak es hatzinor o shehilech sochan." Either he stopped up the drainpipe on the roof so that the rainwater would not run off but would collect around the olives, or he moved the olives about in the water that had gathered. Either one is a deed that shows he was not a bystander to the rain but an active participant in it, and only then is the water b'chi yutan. Being happy on the inside, without any act to express it, does not turn the rain into water that he wanted.