Mikvaos, Chapter 10, Mishnah 2. This chapter deals with the requirement of bi'as mayim: when a keli or a garment is immersed in a mikvah, the water must actually reach every part of it. Our Mishnah applies that principle to items that have an enclosed inner space, and it draws a line: sometimes the water must be able to enter that inner cavity, and sometimes it need not.
First on the Mishnah's list are "hakar vehakeses", a pillow and a cushion. Each of these is an object built around a stuffed interior that an outer covering closes off. When the covering is of leather, "shel ohr", the Mishnah rules that a valid tevilah demands that the mikvah water actually make its way into that inner space. Soaking the outer surface alone accomplishes nothing here; the inside has to be reached as well.
The Mishnah then lists items where the opposite is true. "Keses agula", a round stuffed cushion, sewn shut around its filling. "Vehakadur", a ball, sealed closed with an empty or stuffed space inside. "Veha'umah", a leather shoe form with stuffing inside it. "Hakamia", an amulet: a piece of klaf on which sheimos have been written, sewn into a leather covering. "Vehatefilah", the Mishnah's singular form for tefillin, which likewise has an inner compartment where the parshiyos are housed. Regarding all of these the Mishnah rules "einan tzrichin sheyavou bahem mayim": if they became tamei, they may be immersed without the water reaching inside.
What separates the two lists
Why should a leather pillow or cushion demand that the water enter, while a round cushion, a ball, a shoe form, an amulet and tefillin do not? The answer lies in how each item is used. A pillow and a cushion are made to have their contents put in and taken out: when one wants to launder the covering, the filling comes out and later goes back in. That inner space is genuinely used as a container, even if only for a temporary period, and therefore it carries the status of a beis kibul, a receptacle. A receptacle must be exposed to the mikvah water.
The items in the second list, by contrast, are sewn shut and are meant to remain that way. Their inner space is not a place into which things are inserted and from which they are removed; it is simply closed up permanently. Since that space is not functioning as a receptacle at all, the tevilah is valid without the water penetrating it.
The Mishnah wraps up with a general formulation, "zeh haklal". Whatever the object may be, a garment, a utensil or a leather pouch, if its normal manner of use does not involve putting contents in and drawing them out, "kol she'ein darkah lehachnis ulehotzi", then it goes into the mikvah sealed as it is, "tovlin stumin". Nobody has to open it up or drive the water into its cavity. Everything hinges on a single question: in the ordinary use of this item, does the enclosed space actually serve to hold things, or is it nothing more than a shut-off pocket?