Keilim, Chapter 2, Mishnah 6. Earlier in this chapter we learned that a broken earthenware vessel is not automatically finished as a keli: if the surviving fragment can still hold a certain measure of liquid, it keeps the status of a utensil. Such a fragment, a piece of a vessel that split apart yet still functions as a receptacle, is called a gistira. Our mishnah now adds an essential condition: a fragment can only carry over the status of a keli if the object it came from was already a keli at the moment it broke. There must be something to carry over. If the original object had not yet reached the status of a utensil, its broken piece has no status to retain, and it cannot become tamei.
Nothing is a keli before the work is finished
The principle that decides the case is this: an object acquires the standing of a utensil only when its production has been brought to its end. With earthenware, that end point is the fire. Shaped clay is set into the kiln and baked at fierce heat until it sets into something hard and firm, and that baking is what completes the craftsman's labor and turns clay into a utensil. Until it takes place, the object may already look exactly like a bowl or a jug, but in halachah it is nothing yet.
The mishnah accordingly describes a vessel that split apart while it was still sitting inside the kiln, one of the resulting pieces being large enough to serve as a container. Its words: "Gistira shenimtzeit bakivshan, ad shelo nigmera melachtah, tehorah." A fragment discovered in the kiln, where the vessel came apart at a stage when the fire had not yet done its work and the clay had not yet hardened, stays pure; tumah has nothing to take hold of, because no utensil ever came into being. "Mishenigmera melachtah, temeiah": should the vessel have already been fired through and only afterward cracked, the surviving piece is susceptible. A genuine keli existed and then turned into a gistira, so the standing it had already acquired remains with the fragment.
The titros: an implement for watering plants
The second half of the mishnah moves to a device made for irrigating plants. Its floor is punctured with an array of small holes, while above there is one narrow opening. A person would fill the device with water and seal that upper opening with his fingertip, and the water would stay put rather than draining out below, since no air could enter to take the place of any water that left. He would carry it over to his plants, lift his finger away, and the water would descend through the holes in the floor. This device is called a titros.
Does it qualify as a receptacle? The Tannaim disagree. "Rebbi Eliezer bar Tzadok metaher": in his opinion the device stays pure and tumah cannot touch it, because a floor riddled with holes means it does not truly contain water at all. Water may sit in it for a while, yet a thing left open underneath does not hold its contents in any lasting way, and without such holding there is no receptacle here.
"Rebbi Yose metamei, mipnei shehu motzi tipin tipin": his position is that it is indeed susceptible, on the grounds that the water leaves it only in drops. As he sees it, the device serves as a container at both stages of its use: with the finger sealing the top, the water is genuinely kept inside, and once the finger comes off, the flow is a slow trickle rather than a rush. Something that keeps liquid in and releases it little by little counts as a receptacle, and a receptacle is a keli, and a keli is open to tumah.
That is the thread that ties the whole discussion together: holding liquid makes an object a receptacle, being a receptacle makes it a keli, and being a keli brings with it the possibility of becoming tamei.