Machshirin, Chapter 6, Mishnah 1. The whole masechta turns on one principle: produce becomes susceptible to tumah only when liquid comes onto it with the owner's ratzon. The Torah's words "ki yutan mayim al zera" tell us that the wetting has to be something a person is pleased with. Our Mishnah applies that rule to a very ordinary situation: a man carries his produce up to the roof, and overnight the dew settles on it.
"Hamaleh peiros lagag mipnei haknimah, veyarad aleihen tal, einan b'chi yutan." A person carried his fruit onto the roof out of concern for knimah, infestation by insects, and during the night dew settled on it. The Mishnah rules that this is not b'chi yutan. What stood behind his climbing to the roof was safeguarding the fruit from being eaten by insects, and getting it wet formed no part of his plan at all. The moisture arrived of its own accord, with nothing in his purpose welcoming it, and therefore no hechsher lekabel tumah results.
"Niskaven lechach, harei zeh b'chi yutan." If, however, he did have that intention, meaning that the very purpose of bringing the produce onto the roof was that the dew should fall upon it, then it is b'chi yutan. Now the wetting is something he wanted, he is pleased with the result, and the produce is rendered fit to receive tumah.
The Mishnah continues with a case that draws a firm boundary. "He'elum cheresh, shoteh, vekatan": the ones who carried the produce up were a cheresh (a deaf-mute), a shoteh (one whose mind is unsound), or a katan (a minor). And the Mishnah teaches, "af al pi shechishvu sheyered aleihen hatal, einan b'chi yutan": even in a case where their whole aim in bringing it up was that the dew should settle upon it, the produce is nevertheless not b'chi yutan.
The Mishnah supplies its own reason: "sheyesh lahem ma'aseh ve'ein lahem machshavah." A deed performed by these three is halachically recognized; a thought of theirs is not. See how that works out in practice. Suppose one of them took water and wet the food himself. The food would indeed become susceptible to tumah, because his deed stands, and a deed of that sort suffices by itself to make the wetting count as wanted: the water reaches the food through the act itself, and no one has to investigate what was going on in his mind.
Our case is different. Here they performed an act (carrying the produce up) which then led to liquid arriving on its own. In such a case we must look back and ask what the purpose of the act was, and that is exactly where a cheresh, shoteh, and katan fall short: their intention carries no halachic weight. Whenever the outcome depends on why the produce was brought up, their thinking cannot supply the ratzon that the Torah requires.
So the rule of the Mishnah comes out clearly. Where the purpose behind the act is what determines the halachah, the cheresh, shoteh, and katan cannot render the produce b'chi yutan, and dew that falls on produce they placed on the roof leaves it without hechsher lekabel tumah, even though they had in mind that the dew should fall.