Machshirin, Chapter 6, Mishnah 2. Throughout this masechta the question is whether liquid came onto food with the owner's ratzon, since only liquid that a person is pleased with creates hechsher lekabel tumah. Our Mishnah opens with another roof case, and then moves to a series of practical rulings about food bought in the marketplace.
"Hamaleh es ha'agudos ve'es haktzios ve'es hashum lagag bishvil sheyamtinu, einan b'chi yutan." Someone carried garlic, dried figs and tied bunches of produce onto his roof in order that they should not dry out. Such foods hold moisture of their own, and moisture that belongs to the food itself never creates susceptibility to tumah. The roof is a good place for them precisely because they keep that moisture longer there, which is exactly the owner's purpose. Then dew falls on them. That dew is not b'chi yutan, for it answers no wish of his; on the contrary, it troubles him, since wet produce spoils and he gains nothing from it. His only aim was that the food not lose the moisture already inside it. Consequently no hechsher lekabel tumah has taken place.
The Mishnah proceeds: "Kol ha'agudos shel beis hashvakim temei'in." Every bunch of vegetables sold in the market has the status of tamei. Why? Because sellers of vegetables made a regular practice of sprinkling water over their wares so that they would look crisp and appealing, much as one sees nowadays in a supermarket, where a light mist falls over the vegetable counter at intervals. Water put on for that purpose is applied with complete ratzon, so the vegetables are wet leratzon and become susceptible. Beyond that, customers are forever picking them up and looking them over, and in a crowded market a good number of those customers are themselves tamei, so we work on the assumption that tumah was actually transmitted to the produce.
Our masechta ordinarily uses the term tamei loosely, to convey that the food has become susceptible, that is, that it received hechsher lekabel tumah. This Mishnah is one of the few places where the word is meant exactly as it sounds. The Rav, along with other meforshim, takes the ruling at face value: these vegetables are given a chazakah of real tumah. The practical upshot is that a person who purchases vegetables in the market has to relate to them as genuinely tamei.
"Reb Yehudah metaher belachin." Concerning the fresh bunches, Reb Yehudah dissents and rules them tahor. As he understands the trade, water was sprinkled only on stock that had been lying around for a day or two and was starting to droop. Produce that is truly fresh has no need of such freshening, and no merchant would trouble himself with it, so the fresh bunches may be presumed to have remained tahor. The first Tanna clearly maintains that the sellers sprinkled their entire stock, drawing no such distinction.
"Amar Reb Meir, vechi mipnei mah timu, ela mipnei mashkeh hapeh." Reb Meir explains why even the fresh vegetables were declared tamei: because of saliva. The vegetable sellers would spray the fresh produce from their mouths in order to keep it looking fresh, and that liquid, applied deliberately, is what renders them tamei.
The Rav notes where this dispute makes a practical difference. On the view of the first Tanna, market bunches carry a chezkas tumah universally, the fresh ones included, since water is sprinkled over everything. On Reb Meir's view, the outcome hinges on the custom of the place: the fresh produce is tamei only where the vendors spray it from their mouths, whereas somewhere that this is not the practice, fresh vegetables would not be assumed tamei.
"Kol hakemachin veha'salasos shel beis hashvakim temei'in." All the flours and the fine flours of the marketplace are tamei. When a person buys flour, one of his concerns is that it be clean, and to produce clean flour the grain must be soaked in water so that the bran and other matter can be separated out. That soaking is done fully beratzon. Once again the word tamei here means actual impurity: the grain was wet with ratzon and thus received hechsher lekabel tumah, and since people who may themselves be tamei could well have handled it, all flour purchased in the marketplace carries a chezkas tumah.
The Mishnah concludes: "Hachilka vehatargis vehataski temei'in bechol makom." These are three kinds of groats from which a porridge was cooked. To prepare them, the kernels of grain are first soaked in water and afterwards broken apart. The Rav distinguishes the three by how far the kernel is broken: in chilka the grain is split in two, in targis into three pieces, and in taski into four.
Since there is no way to prepare such groats other than by soaking, their assumed status of tumah attaches to them regardless of their source, whether they were bought in the market or received from someone's private home. That is not so with the flour of the previous ruling, where the desire for clean flour, and therefore the soaking, is characteristic of what is offered for sale, while flour ground in a private house may never have been soaked at all. With groats no such distinction exists: grain simply cannot be broken up in this way unless it was first put in water, and so these three are treated as tamei wherever one finds them.