Uktzin, Chapter 2, Mishnah 8. Our chapter has been occupied with the shiurim of tumah: a food that has become tamei can pass its tumah on to other foods only when there is a k'beitzah of ochel, and much of the chapter asks which parts of a given food are counted toward that k'beitzah and which are not. This Mishnah adds a new angle to the question. Here the food itself is present, but its bulk is not entirely solid: there is sap inside it, or air, or the food has changed size in the pot. Do we measure the item as it stands before us, or do we first reduce it to what is genuinely food?
Leaves of leeks and onions
The Mishnah begins with "alei vetzalim uv'nei vetzalim", the long green leaves that shoot up out of leeks and onions. These leaves are not flat and dense; they are tubes, and what fills them varies.
The first case: "im yesh bahem rier", if there is a sap inside them, a moisture that you can press out as a bit of juice. In such a case, "mishta'arin kemo shehein", they are measured exactly as they are. We do not squeeze the sap out first. Even if pressing the leaves would leave you with less than a k'beitzah, as long as the leaves in their natural state amount to a k'beitzah, that is the shiur, and these leaves join together with the rest of the vegetable to complete it.
The second case: "v'im yesh bahem chalal", if what is inside the leaf is not sap at all but simply empty space, a hollow shaft with nothing in it. Here the halachah is "mema'ech chalalan": before measuring, you press the leaf together so that the hollow is closed off. Air is not ochel, and it may not be counted toward the k'beitzah. Once the empty space is squeezed shut, whatever bulk remains is what we measure.
Spongy bread
The same distinction is now applied to bread. "Pas sefgoniyos" is spongy bread, the sort of loaf that would occupy noticeably less room if you were to press down on it. Nevertheless, "mishta'eres kemo shehee", it is measured just as it is. Its sponginess is simply the nature of the bread and not a foreign emptiness sitting inside it. Most of our own breads are like this: press down on a slice and you can compact it considerably, yet nobody would say that the loaf is smaller than it looks. Its full volume counts toward the shiur of tumah.
But "im yesh ba chalal", if this particular loaf has an actual pocket in it, a bubble of air trapped inside the dough, then "mema'ech chalalah". That pocket is not part of the bread. You press the loaf so that the bubble collapses, and only then do you compute whether there is a k'beitzah.
Meat that swells and meat that shrinks
The Mishnah closes with a different sort of change in size, one produced by cooking. "Basar ha'egel shenispach": the meat of a young calf sometimes expands in the pot and comes out larger than it went in. "O basar zakein shenisma'eit": the meat of an older animal often does the reverse and shrivels while it cooks. In both cases the ruling is "mishta'arin kemo shehein", we measure them as they are now, in their present state, and that is the size we work with for tumah.
The rule cuts both ways. If the calf's meat at this moment amounts to a k'beitzah, it can convey tumah to other foods, even though before it was cooked it was smaller than a k'beitzah. And if the older animal's meat is now less than a k'beitzah, it cannot convey tumah to other foods, even though when it began it was a full k'beitzah.
Seen as a whole, the Mishnah teaches us to take food as it actually presents itself: sap belongs to the leaf, sponginess belongs to the loaf, and cooked meat is judged by what lies on the table now rather than by what it once was. Only genuine emptiness, a hollow shaft or an air pocket, is excluded, because empty space was never ochel and can never help make up the k'beitzah.