Keilim, Chapter 2, Mishnah 3. Our Mishnah presents a list of utensils made of earthenware that stand outside the laws of tumah entirely. Some of them are simply flat, with no hollow at all. Others do have a hollow, but they were never designed to hold anything, or they serve as lids and covers rather than as containers. The Mishnah opens: "Hatehorim shebekeli cheres", these are the earthenware items that remain tahor.
Flat Utensils With No Interior
"Tavla she'ein la lizbez", a serving tray that is completely flat and has no raised rim running around its edge. "Umachtah perutzah", a coal shovel that is open on its sides, with no walls to enclose what is placed on it. "Va'avuv shel kala'in", the flat pan used by those who roast and toast grain. Picture something along the lines of the peel used for a pizza: a surface, and nothing more.
None of these can contract tumah, because none of them is a receptacle, and an earthenware utensil that has no cavity to hold anything simply has no capacity for tumah. The source is the wording of the Torah itself, "asher yipol meihem el tocho", which speaks of a dead sheretz falling into the utensil. The verse assumes a utensil with an inside. Where there is no interior, there is no possibility of tumah.
Utensils With a Hollow That Was Never Meant to Hold
Next come objects that may end up holding something even though holding was never their purpose. "Vesilonot, af al pi kefufin, ve'af al pi mekabbelin": earthenware conduits. Even where the pipe curves along its course, and even where that curve actually traps water and keeps it there, the pipe stays tahor. A conduit was never manufactured as a container; it was made to move water from one place to another, and that is how it is judged.
A chavkav is a high earthenware covering made to sit over a basket of bread: "Vechavkav she'asahu lesalapat". A tefi is ordinarily a vessel for oil, and the Mishnah here speaks of one that its owner fitted out to serve as a covering over grapes: "Utfi shehitkino la'anavim". Both of them are capable of holding, and the tefi even began its life as a container. Still, the two are tahor, since neither one counts as a receptacle now. Their job is to cover, and once covering is what a utensil is for, whatever room it happens to have inside does not grant it the status of a container.
A Hollow That Serves the User, Not the Contents
"Vechavit shel sayatin", a sealed empty barrel used by a swimmer as a flotation device. "Vechavit defunah bishulei hamachatz", a large, heavy earthenware utensil that has an indentation formed in it so that a person can insert his hand and gain leverage to lift and carry the piece.
Neither of these becomes tamei. The indentation in the heavy vessel exists to help a person carry the vessel, not to hold anything within it. And as for the swimmer's barrel, the air sealed inside it is precisely the point, yet air is not contents. The barrel was never meant to be filled with anything, only to remain hollow so that it will float. That is not the function of a genuine receptacle.
Furniture, a Boat, and a Lampstand
"Vehamitah vehakisei vehasafsal vehashulchan": ordinary household furniture, a table, a bench, a chair, a bed. True, people sit and recline on them and set objects down on them, yet none of them becomes tamei, for from the standpoint of tumah what we have here is a flat surface with nothing contained within it. As for midras tumah, which normally takes hold of whatever is designated for sitting and lying, bear in mind that the furniture under discussion is made of earthenware, and the Torah never applied midras to earthenware at all.
"Vehasafinah", a boat. Here the reasoning is different. A boat is very much made to hold: it carries people and it carries cargo. Nevertheless the Torah removes boats from the laws of tumah, as is derived from a pasuk in Mishlei.
"Vehamenorah shel cheres", an earthenware menorah, meaning the earthenware base that serves as the stand supporting the lamp. It too remains tahor, since it is essentially a flat support and is not regarded as a receptacle.
"Harei eilu tehorim", all of these, when they are made of earthenware, are tahor and cannot contract tumah, each for the reason explained in its own case.
Zeh Hakelal: Tamei Liquids and the Outside of a Utensil
From here the Mishnah moves into a neighboring subject: liquids that are tamei, and what they are able to do to a utensil. According to Torah law, a tamei liquid has no power at all to render a utensil tamei. The tumah involved is a decree of the Chachamim, who ruled that such liquids do transmit tumah to keilim. To keep it clear that we are dealing with rabbinic law and not Torah law, they shaped their decree in an unusual manner: contact with the outer wall leaves the tumah on the exterior alone, and the inner part of the utensil is unaffected.
Earthenware is included in this rabbinic tumah, and the general rule with which our Mishnah concludes marks its limit: "Zeh hakelal: kol she'ein lo toch bekeli cheres, ein lo achorayim". Where a vessel of earthenware has no inner space, there is no exterior to speak of either. Practically, then, a flat earthenware item does not even pick up the rabbinic tumah of its outer side upon contact with a tamei liquid.
Why should it matter so much whether a given tumah comes from the Torah or from the Chachamim? Because the halachah of burning depends on it. Terumah and kodashim that contracted tumah are burned only where that tumah is d'oraita. When the tumah rests on a rabbinic decree alone, they are not burned, for that would mean ruining kodashim in a case where the Torah itself imposed no such demand.