Keilim, Chapter 17, Mishnah 13. After a stretch of mishnayot that dealt with measurements and definitions, our mishnah returns to the central subject of the masechet itself: which utensils are capable of receiving tumah and which remain tahor no matter what touches them. The rule taught here draws a line not by shape or by size, but by origin: was the raw material of this utensil taken from the sea or from the land?
The mishnah opens with a sweeping statement: "Kol shebayam tahor." Anything that comes from the sea is tahor. A utensil fashioned from the hide of a sea creature simply does not enter the system of tumah at all.
Where the rule comes from
The Torah, in listing the utensils that a sheretz renders tamei, speaks of a utensil of wood, a garment, leather, or sackcloth, that is, any utensil with which work is performed. Notice that leather appears in the same breath as a garment. That pairing is what teaches the halachah. A garment is by definition woven from material that grows on dry land, from flax or from wool. Since the pasuk places leather alongside it, the leather intended by the Torah is likewise leather that came from the land, from a creature of the land. Hide taken from a creature of the sea is thereby excluded, and a utensil made from it stays tahor.
The exception: kelev hayam
The mishnah adds: "chutz mikelev hayam," except for the sea dog. There is one sea creature whose hide does receive tumah, a dog-like creature of the water. The Chachamim suggest that what is being described may be the otter.
And why should this creature be treated differently from every other inhabitant of the sea? The mishnah gives the reason: "mipnei shehu boreiach layabashah," because it flees to the dry land. When danger threatens it, this creature escapes onto the shore. Its instinct in a moment of peril reveals where it truly belongs. Though it lives in the water, it is classified as a creature of the land, and its hide takes on tumah accordingly. This, the mishnah concludes, is divrei Rabbi Akiva, the opinion of Rabbi Akiva.
When sea material joins land material
A second case now follows, one in which material of the sea can indeed become tamei. Picture someone who forms a utensil out of that which grew in the water, "hagadel bayam," and then fixes onto it a piece of something that grew on dry land, "hagadel ba'aretz." How substantial must that land-grown addition be? Hardly anything at all: "afilu chut, afilu meshichah," a mere thread will do, or a cord or ribbon. The test is the manner of the attachment. If the two pieces are fastened in the way that would render them a single unit for the laws of tumah, such that had both components come from the land, tumah reaching one would carry over to the other, then the ruling is "davar shehu mekabel tumah, tamei": this utensil is susceptible, and it becomes tamei.
The principle behind this is the law of chibur, of joining. Two utensils fastened together in a permanent manner are no longer two objects but one larger utensil, and once they are one, tumah that reaches either part reaches the whole. Here, the thread or the ribbon binds the sea hide to the land material in exactly the way that would unite two land utensils. The result is a single object, and when one portion becomes tamei the other becomes tamei along with it.
So we arrive at a striking outcome. The greater part of this utensil was formed from the hide of a sea creature, material that on its own could never receive tumah at all. Yet because it has been permanently joined to material of the land, the sea portion is batel, nullified to the land leather, and the entire utensil is judged as a utensil of the land. What comes from the sea stands outside tumah when it stands alone; once it is bound into something of the land, it is drawn into the land's law.