Keilim, Chapter 4, Mishnah 2. The previous mishnah discussed an earthenware barrel that actually came apart, and asked whether what remains still counts as a keli that can receive tumah. Our mishnah moves one step earlier: the barrel has not fallen to pieces, it has only become cracked. The question now is how badly a vessel must be cracked before the halachah treats it as though it were already broken.
The mishnah begins with the barrel: "chavit shenitro'a'ah", an earthenware barrel in which cracks have opened up so that its solidity is gone, "ve'eina yecholah lehitaltel bachatzi kav gerogrot", so that it can no longer be carried about with a half kav of gerogrot (dried figs) inside it, "tehorah", it is tahor.
A half kav of gerogrot is the quantity the Sages use to represent the amount of food a person eats at one meal, and here it becomes the test of the barrel's usefulness. Put that modest load of food inside and try to lift the barrel and carry it: does it hold together, or does it give way? If the cracking has left it too weak even for this, the barrel has stopped functioning as a barrel. A vessel that no longer works is not a keli at all, so it cannot take on tumah, and any tumah it already carried departs from it.
The mishnah then applies the same kind of question one level further down, to a gisterah. A gisterah is a broken shard of earthenware that is still large enough and shaped well enough to be used, so it retains the status of a keli even though it is a remnant of something larger. Now the mishnah asks what happens when that remnant itself becomes cracked.
So the mishnah teaches: "Gisterah shenitro'a'ah ve'einah mekabelet mashkin", a gisterah in which cracks have formed so that liquid poured into it drains away and does not stay put, is tahor. This is so "af al pi shehi mekabelet ochalin", notwithstanding that dry food would still sit in it perfectly well, and the mishnah supplies the reasoning: "she'ein shirayim leshirayim", a leftover has no leftover of its own.
The principle is this. A person will place a shard underneath a leaking vessel to catch what drips out. But nobody bothers to set a second broken shard beneath a leaking shard in order to save the liquid escaping from it. A remnant does not get its own remnant. Once the gisterah leaks, it is simply discarded.
Behind this lies the way a gisterah is actually used: it serves for liquids, not for solid foods. As long as it holds liquid, it is worth keeping and is a keli. The moment it begins to leak, its owner throws it away, and the fact that solid food could still be placed in it does not preserve its status. Therefore a gisterah that can no longer hold liquid is no longer a keli, and it cannot become tamei.