Keilim, Chapter 26, Mishnah 3. This chapter deals with articles made of leather and asks, item by item, which of them count as keilim that can contract tumah and which do not. The governing principle is whether the object serves as a receptacle, something that holds and contains, or whether it is merely a covering that protects the body. Our mishnah runs through a list of leather items worn by workers and rules on each one.
The mishnah opens: "kaf shel loktei kotzim, tehorah", the pad of those who gather thorns is tahor. In the times of the Mishnah, a worker who handled thorny branches would strap a leather pad onto his hand so that his skin would not be torn and scratched. Since the entire function of that pad is to shield the hand, and it is not designed to hold anything inside it, it is not considered a receptacle and therefore cannot become tamei.
"Hazon vehabirkiyar, teme'in". The zon is a leather belt, and the birkiyar is a leather knee guard. Both of these can become tamei, because each one does have a space that holds something. The belt is fashioned with a pocket in which coins could be kept, and the knee guard has a hollow section on top which was stuffed with padding so that it would rest more comfortably on the knee. Once there is a place that receives, we are dealing with a keli.
The list continues with the sharvulim: "vehasharvulim, teme'im". A laborer would draw these leather tubes over the sleeves of his clothing, so that whatever substance he was handling would not spoil the garment underneath. A sleeve like this is open at either end and encloses nothing, so mide'oraita it has no receptacle and ought not to be susceptible at all. The chachamim nonetheless ruled that it does take on tumah, since anyone looking at it would take it for a genuine keli.
"Vehaperaklimin, tehorin". These are headbands that a person would tie around his forehead. They are tahor, since there is no receptacle in them whatsoever.
The mishnah then turns to gloves and declares the whole category clean: "vechol beit etzba'ot, tehorot". The ruling can sound puzzling, for a glove plainly has a hollow interior into which the fingers and hand slide. That space, however, was never fashioned in order to store anything inside; it is no more than the form the leather must take in order to wrap around a hand. The glove is therefore judged like any flat piece of leather, which has no receptacle and remains outside tumah.
To this there is one exception: "chutz mishel kayatzin, mipnei shehi mekabbelet et ha'og", except for the glove of fruit gatherers, because it does hold the og, the sumac berry. A worker out gathering fruit would come across sumac plants along the way, pick some of the berries and store them inside his glove. That glove genuinely serves as a container to a certain degree, and so it is susceptible to tumah.
Finally the mishnah considers what happens once such a glove is damaged: "nikre'ah", it tore. The measure it gives is whether the glove still functions as a container for the berries: "im einah mekabbelet", if it can no longer take in "rov ha'og", the greater part of the sumac it used to hold, then "tehorah", it is clean. A rip of that extent robs the glove of the single purpose that had turned it into a receptacle, and with that purpose gone its susceptibility to tumah departs as well.
Running through the list, we see one consistent measure at work. Leather that guards the body, whether on the hand, the forehead or over the sleeve, remains outside the category of keli, while leather shaped to receive and hold, whether coins in a belt, padding in a knee guard or berries in a gatherer's glove, enters it. And the moment that holding function is lost, as in the torn glove, the status of tumah departs along with it.