Uktzin, Chapter 3, Mishnah 3. This chapter is built around two conditions that can bring an item into the world of tumas ochlin, the tumah of foods. The first is machshava, a person's mental designation of something as food, which is needed when the item is not ordinarily eaten. The second is hechsher, contact with water or another liquid, which is normally required before any food can become tamei. Our Mishnah teaches a category of items where the second condition falls away entirely, because these items carry their own tumah inside them.
The Mishnah opens: "Nivlas beheima temeiah bechol makom, venivlas ha'of hatahor bakfarim, tzrichin machshava ve'einan tzrichin hechsher." A non-kosher animal that died, in any location at all, together with a kosher bird that died without shechitah when it is found in the villages: both of these require machshava, yet neither requires hechsher.
Why do these need machshava? Because they are not the kind of thing people ordinarily place on their table. A neveilah of a non-kosher animal is not fit for regular human consumption, and a kosher bird that was not shechted is likewise not treated as food in the villages, where there are not many people around who would want it. Something that is not normally eaten only enters the category of ochel when a person decides that it will serve as food. Once that decision has been made, however, no hechsher is needed. These carcasses do not have to come into contact with liquid, because a neveilah already possesses its own tumah, a tumah that works from within the item itself and does not wait for water to open the door.
A second list now follows: "Nivlas beheima tehorah bechol makom, venivlas ha'of hatahor vehacheilev bashvakim, einan tzrichin lo machshava velo hechsher." A kosher animal that died without shechitah, in any location at all, along with a kosher bird that died and with cheilev, the fat of a kosher animal that the Torah forbids, when these are found in the marketplaces: they require no machshava and no hechsher either.
Here the difference is the setting and the item. In the marketplaces, where crowds of people are passing through, such items are regarded as food; there are always takers, and so they carry the status of ochel on their own. No special designation by an individual is required to turn them into food, since they already are food in the eyes of the public. And once again no hechsher is required, since the neveilah tumah comes from within and does not depend on liquid at all.
The Mishnah closes with the view of Rabbi Shimon: "Rabbi Shimon omer, af hagamal veha'arneves vehashafan vehachazir." Rabbi Shimon adds the camel, the arneves, the shafan and the pig to the list of items that need no machshava. Although these are non-kosher animals, they are widely eaten, and an item that many people consume is already established as food. Therefore, according to Rabbi Shimon, no individual act of designation is needed to make them into an ochel.
The thread running through the entire Mishnah is a single principle: whether an item requires machshava depends on how people relate to it, while the exemption from hechsher flows from the nature of neveilah tumah, which needs no outside help to take hold.