Uktzin, Chapter 2, Mishnah 7. Much of Maseches Uktzin turns on the question of chibbur: when are two things counted as one single unit for the laws of tumah, and when have they already parted ways? Our Mishnah gathers a series of cases in which something wraps around food or holds items together, and asks at what stage of breaking or unraveling that unity comes to an end.
The Mishnah opens with an egg: "Beiah megulgeles ad sheyagis." A lightly boiled egg is still soft inside, and its shell is what holds the contents in place and protects them. Therefore the shell and the egg are one unit for tumah, and they remain so until sheyagis, until he taps and opens a small area of the shell in order to pour out the contents from there. From that moment the shell no longer functions as a shomer, a protective guard for the egg, since the egg can now seep out through the opening.
A hard boiled egg is judged differently: "Veshelukah ad sheyerasseis." Its contents have set solid and are in no danger of running out, so puncturing a bit of the shell accomplishes nothing; the shell goes on covering the egg and keeps its standing as a shomer. It holds that standing until sheyerasseis, until the shell has been crushed into numerous fragments. And even this requires a further point, since a thin skin lining the interior of the shell keeps those fragments clinging in place. While nothing more than a crack has appeared, the shell is still doing its protective work. Only after the whole shell has come apart into many small pieces, even while those pieces continue to encase the egg, does its role as a shomer come to an end.
From the egg the Mishnah moves on to a bone: "Etzem sheyeish bo moach, chibbur ad sheyerasseis." Marrow inside a bone is something a person eats: he spills it onto his food as a rich sauce, or he draws it out of the bone by sucking. The bone is thus acting as the vessel and protector of edible matter, which gives it the standing of a shomer, and in that capacity bone and marrow are reckoned as a single entity: the bone takes on the halachic character of food, and its bulk joins the marrow's in reaching the amount needed for tumah. This holds true until sheyerasseis, until the bone has been smashed into many fragments and can no longer contain or shield its contents.
Next comes the pomegranate: "Harimon sheparso, chibbur ad sheyakish alav bekaneh." A pomegranate he has broken open still has its numerous seeds fixed to the rind. While that attachment holds, the fruit counts as chibbur, one item. The connection ceases sheyakish alav bekaneh, once a reed is used to strike the fruit so that the seeds are dislodged from the rind. The seeds may well remain in place, tucked in among the membranes, and nevertheless they are no longer reckoned as one entity together with the pomegranate.
The Mishnah then leaves food behind and brings two parallel cases from the world of garments, where a thread is what creates the unity. "Kayotzei vo shel hakovsin": when people would send their clothing out to the launderers, a thread would be run through the various items so that all of one person's garments stayed together and would not be mixed up with someone else's wash. The laundering was done with the whole group strung on that single thread, and the halachah treats them accordingly as connected.
And "habeged shetofro bekilayim": two garments, one of linen and one of wool, that have been sewn to one another with a common thread, so that they must eventually be cut apart. Here too the ruling is chibbur: as long as that thread joins them, they are one item, so that if one of the garments becomes tamei, the second garment attached to it by the thread becomes tamei as well. That lasts ad sheyaschil lehatir, until he begins to undo the knot and the stitching. From the moment the untying has begun, we regard them as disconnected and count them as two separate items, and tumah touching one no longer affects the other.
Case after case, the Mishnah teaches the same principle from a different angle: a covering or a thread creates a single halachic unit only so long as it is genuinely doing its work of guarding or holding together. Once the shell is crumbled, the bone shattered, the seeds knocked loose or the stitching started to be opened, what looked like one item has already become two.