Makhshirin, Chapter Three, Mishnah Five
1) If one moistened [produce] with drying clay:
a) Rabbi Shimon says: if there was still in it dripping liquid, it comes under the law of if water be put;
b) But if there was not, it does not come under the law of if water be put.
2) If one sprinkled his threshing-floor with water, he need not be concerned lest wheat be put there and it become moist.
3) If one gathered grass with the dew still on it in order to moisten wheat with it, it does not come under the law of if water be put,
a) But if his intention was for this purpose, it does come under the law of if water be put.
4) If one carried wheat to be milled and rain came down upon it and he was glad of it, it comes under the law of if water be put.
a) Rabbi Judah said: one cannot help being glad of it.
i) Rather, [it comes under the law] only if he stopped [on his way].
Section one: Rabbi Shimon says that in this case the produce is susceptible only if the drying clay is sufficiently wet that it actually drips. If not, it is not susceptible.
Section two: The assumption is that by the time he puts wheat down on his threshing floor it will already be dry enough such that the wheat will not become moist and susceptible to impurity. This is probably because a threshing floor is outside, while the house is obviously inside.
Section three: The intention here was to moisten the produce with the moisture from the grass. The dew was incidental to his intentions so it doesn’t cause the produce to be susceptible. However, if he intended to use the dew to make the produce moist, it does become susceptible.
Section four: In this case the produce is only susceptible if he was happy that the rainwater came down upon it. The produce is not automatically susceptible because he didn’t put it there actively.
Rabbi Judah says that there is simply no way a person won’t be happy if the rainwater comes down on his wheat. I’m assuming this makes it easier to mill. Therefore, we can’t gauge its susceptibility by whether he is happy. Rather, in order for it to be susceptible he must actively do something to make it susceptible, such as stop walking so that more rain can come down upon the wheat before he brings it to the mill.