Keilim, Chapter 20, Mishnah 1. This mishnah deals with keilim that serve two purposes at once: they are containers, made to hold things inside them, and they are also the kind of object that a person will lie down or sit upon. Because of this double role, two separate channels of tum'ah are open to them. They can receive tum'ah in the ordinary way, through contact with something impure, and they can also receive tum'at midras, the tum'ah that attaches to something designated for lying and sitting.
The Mishnah begins: "hakarim vehakesatot, vehasakin vehamarsupin", pillows and mattresses, sacks and packing cases. Every one of these is a container of some sort, and every one of them is also something people will stretch out on or sit down on. "Shenifchatu", if they became damaged to the point that they can no longer serve as containers, "harei eilu temei'im midras", they nevertheless remain capable of receiving tum'at midras. The container function is finished, so tum'ah through contact with an impure object no longer takes hold of them. But a person can still sit or lie on them exactly as before, and that use alone keeps them within the category of midras.
The Mishnah then turns to a list of keilim where the measurement is the deciding factor. "Hakelustar arba'at kabin", a feed basket used for animals that holds four kabin; "haturmal chameshet kabin", a shepherd's pouch that holds five kabin; "hakerit se'ah", a small leather sack that holds a se'ah; "hachemet shel shiv'at kabin", a leather water bottle that holds seven kabin. When these keilim are of these capacities or larger, they are subject to tum'at midras.
Why does the size matter so much? Because at these capacities the item is genuinely functioning as what it was made to be, and alongside that function people will also settle down on it and rest. A kli of this size is filled, closed and left standing, and a person is happy to sit or lie on it. Smaller than the listed measure, however, the kli can no longer do its intended job. A feed basket that does not hold four kabin cannot properly feed the animals; a shepherd's pouch smaller than five kabin will not take his supplies. Such a kli ends up being used for storing odds and ends around the house, and household items are constantly needed, so the kli is being opened again and again. Nobody would lie down on something he must keep getting off of. That use simply never develops, and so a kli of less than the stated size is not subject to tum'at midras.
"Rabbi Yehuda omer: af harifsa vehamizvada kol shehen, harei eilu temei'in midras." Rabbi Yehuda holds that the rifsa, a pouch in which spices are carried, and the mizvada, the satchel a person takes along on the road, are open to tum'at midras no matter what their capacity may be, even the smallest ones. His reasoning is that people really do stretch out on such bags. The Tanna Kamma rules otherwise: what is packed in these bags is easily crushed, so one takes care not to rest his weight on them and spoil the contents, and for that reason they never enter the category of midras at all.
The Mishnah closes with a ruling that covers the whole list: "vechulam shenifchatu, tehorim." Any of these keilim, the feed basket, the shepherd's pouch, the leather sack, the water bottle, that became damaged so that it can no longer hold what it is meant to hold, is tahor and cannot receive tum'ah at all. And the Mishnah gives the reason: "mipnei shebatel ha'ikar batla tefeilah", once the primary function has been nullified, the secondary function is nullified along with it. For these keilim, being a container is the essential use, and lying upon them was only an accompanying, secondary use. When the essential use is entirely gone, nothing is left to carry the secondary use either. Such an item is simply a broken kli, and no form of tum'ah, neither tum'ah through contact nor tum'at midras, applies to it any longer.