Uktzin, Chapter 3, Mishnah 5. This chapter explores the question of what counts as an ochel, a food, for the halachos of tumah. In several of the earlier mishnayos, an item that people do not ordinarily eat could be drawn into the category of food through machshava, a person's mental designation. Our Mishnah presents a group of items where thought alone is not enough.
The Mishnah teaches: "Lulvei zeradim vehachazered va'alei halof hashoteh, einan mitam'in tumas ochlin ad sheyimtoku." The young shoots of the zerad, which the meforshim identify as a service tree, the chazered, explained as a species of garden cress, and the leaves of the wild lof, the wild arum, do not become tamei with the tumah of foods until they are sweetened.
These plants are extremely bitter, and in their natural state a person does not eat them. Nevertheless, they are not worthless: people pickle them, or they cook them together with sweetening agents, and only then do they become palatable. The Mishnah rules that only at that point, once they have actually been sweetened, do they enter the category of ochel and become susceptible to tumas ochlin.
The meforshim draw out the principle at work here. Because of their inherent bitterness, these items are further removed from the status of food than the cases discussed earlier in the chapter. For them, a machshava, a decision in a person's mind that this will serve as food, does not suffice. They require a ma'aseh, a concrete action that changes the item itself and removes the bitterness, and only such an action can make them into a food.
A final ruling is appended in the name of Rabbi Shimon, who adds one more type of produce to this list: pakuos. In his view they are "kayotzei bahen," governed by the same rule as the plants just mentioned. What are pakuos? The meforshim offer two identifications: a bitter sort of melon, or a gourd that grows wild in the fields and is likewise bitter to the taste. Either way, the point is identical. Such produce is fit for eating only after it has been placed in brine and sweetened, and until that treatment is carried out it stands outside the category of food, no matter what a person has resolved in his mind to do with it.
The Mishnah therefore marks a clear line: where an item is merely uncommon as food, thought can elevate it; where the item is intrinsically inedible because of its bitterness, only a real act that sweetens it will bring it into the world of ochel and its tumah.