Keilim, Chapter 17, Mishnah 14. Our Mishnah takes a striking approach to the laws of tumah: instead of listing materials one by one, it walks through the six days of creation and asks, day by day, whether the things brought into being on that day can serve as raw material for a vessel that becomes tamei. Some days produced materials that receive tumah; other days produced nothing that can ever become tamei.
What came into existence on that opening day? Heaven and earth. Of the first day's creations the Mishnah declares "yesh bo tumah," there is tumah in it. Whatever is taken from the ground falls under this heading: a clay pot is formed from earth, and metal too is extracted from the ground, and utensils of both sorts can become tamei.
"Basheini, ein bo tumah" - in what was created on the second day there is no tumah. The second day brought the expanse of the sky, and there is simply no such thing as a utensil fashioned out of the sky.
"Bashelishi, yesh bo tumah" - in what was created on the third day there is tumah. The third day produced the trees, and wooden vessels are certainly capable of becoming tamei.
"Barevi'i u'vachamishi, ein bahem tumah" - the creations of the fourth and fifth days carry no tumah. Day four is when the luminaries were set in place: sun, moon and stars. Day five brought the creatures of the water and the creatures of the air, fish and fowl. A utensil produced from either one is tahor. For sea life this is the familiar rule that whatever grows in the sea does not receive tumah; the novelty here is that the winged creatures are governed by the same principle, so that a utensil fashioned out of a bird also remains tahor and cannot contract tumah.
This ruling does not follow the view of Rabbi Akiva. Rabbi Akiva held that the kelev hayam, the sea dog (also called kelev hamayim, which may well be the otter), is treated as a land animal, since it flees from danger onto dry land. Following that logic, Rabbi Akiva would say the same about birds, which also spend part of their lives on land: any creature that spends time on land can become tamei, and so utensils made from birds would be included as well.
The Mishnah then records two exceptions: "chutz miknaf ha'oz u'vetzat hana'amit hametzupah," except for the wing of the osprey and the coated egg of the ostrich. Although these come from birds, creatures of the sky, they were given the capacity to become tamei by rabbinic decree.
Why these two? Regarding the wing of the osprey, the Chachamim were concerned that vessels made from its wings could be mistaken for vessels made from the bones of land animals, since the two look very much alike. Regarding the ostrich egg, the decree applies specifically to a container made from such an egg that was coated to strengthen it. The ostrich egg is exceptionally strong to begin with, and even without any coating it would endure; all the more so, then, once it has been coated, it has genuine durability. Other eggs are different: they achieve strength only through the coating, so on their own they have no kiyum, no lasting existence, and therefore the Chachamim did not extend the decree to the eggs of other birds, which have no durability at all.
The Mishnah then presents a dissenting voice. Rabbi Yochanan ben Nuri challenges the first of these two exceptions, asking: "mah nishtanah knaf ha'oz mikol haknafayim?" What sets the osprey's wing apart from wings generally? To his mind no distinction can be sustained. Utensils are regularly produced from the feathers of many species of bird, not the osprey alone, and if a decree was issued for the one it should equally govern the others. The earlier reasoning leaned on how closely the osprey's feathers and bones resemble those of land animals; Rabbi Yochanan ben Nuri responds that this concern is present with anything made from a bird, and the decree therefore belongs across the board.
Finally: "v'chol shenivra bayom hashishi tamei" - everything created on the sixth day is subject to tumah. The sixth day brought forth the land animals and man. A utensil fashioned from their hides or their bones is fully capable of becoming tamei.
In one elegant sweep, then, the Mishnah has mapped the entire subject of tumah onto the order of creation: earth, metal, wood, hide and bone receive tumah, while sky, luminaries, fish and birds do not, with the osprey's wing and the coated ostrich egg standing as rabbinic exceptions, and Rabbi Yochanan ben Nuri arguing that those exceptions should never have been exceptions at all.