Keilim, Chapter 24, Mishnah 5. This chapter is built on a single repeating pattern: the Mishnah names one type of object, and then shows that the very same object can fall into three completely different levels of tumah, depending only on what people do with it. Here the object is the tarbus.
We begin with the Mishnah's own words: "Shlosha tarbusin hein", meaning that the Mishnah is about to count three varieties of tarbus. What is a tarbus? A piece of raw animal skin that has been shaped for use. And one detail must be clear before anything else: this piece of leather forms no cavity whatsoever. Nothing can be contained within it, since it amounts to nothing more than a spread-out sheet of hide. That single fact stands behind every ruling in the lines that follow.
The tarbus of the barbers
"Shel saparim tamei midras", the tarbus that belongs to the barbers is susceptible to tumat midras. These were the practitioners who also performed bloodletting, and the hide was placed there specifically so that the person undergoing the procedure would have a place to sit. Since it was designated from the start for sitting, it carries the status of an object made for sitting, and a zav who sits on it renders it a midras, an av hatumah.
The tarbus used as a table
"She'ochlin alav tamei tumat met", the tarbus that people eat off of is susceptible to tumat met. Here we need a general principle. A flat object of wood or of leather that has no receptacle, a pashut, is not susceptible to tumah on the biblical level at all. Nevertheless, the Sages decreed tumah on such an object when it functions in the service of people, and when it also serves other objects that themselves serve people. A table meets both descriptions: it serves the people who eat from it, and at the same time it bears the food and the vessels which in turn serve those people. For that reason this tarbus can contract corpse tumah even though it holds nothing inside.
But it does not receive tumat midras, because it was never intended for sitting. If someone were to settle himself down on it, the people around him would tell him to get up, since as long as he is sitting there they cannot use it for their meal. An object that people push you off of is not an object made for sitting.
The tarbus of the olives
"V'shel zeitim tahor mikelum", the tarbus used for olives is completely pure. After the harvest the olives were spread out on it and left to soften before being taken to be pressed into oil. This hide has no receptacle, and it does not stand in the service of people at all: its whole function is toward the olives themselves. Lacking both a receptacle and a role in serving a person, it remains outside the system of tumah entirely.
Three identical pieces of hide, then, and three different halachot: one an av hatumah through midras, one susceptible only to corpse tumah by rabbinic decree, and one pure from every form of tumah. The leather never changed. Only the use to which people put it did.