Keilim, Chapter 27, Mishnah 5. Our chapter deals with cloth, sack and leather and with the sizes and conditions that allow such items to receive tum'ah. This mishnah takes up a related question: what happens to a utensil that has already worn out from the job it was made for, when its owner now wants to use it as something to sit on? Sitting and lying are the doorway to tum'at midras, the tum'ah transmitted by a zav or a niddah to whatever is designated for supporting a person's weight, so the question is whether an item's second career as a seat is enough to make it susceptible.
The mishnah opens: "beluyei nafah uchvarah shehitkinan lishivah", worn out remnants of a sifter and a sieve that the owner prepared for sitting. A nafah and a kevarah are the implements used for sorting, separating the fine from the coarse. Once they have worn out they can no longer do that work, and the owner decides to put them to a different use altogether and sit on them.
"Rabbi Akiva metamei", Rabbi Akiva rules that they can now become tamei with tum'at midras. In his view the owner's intention is decisive: once he has decided that this worn out sifter is his seat, it has become a seat, and a seat is subject to midras.
"Vachachamim metaharin ad sheyikatzeya", the chachamim declare them tahor unless he trims them so that they are genuinely fit to be sat upon. For the chachamim, thought alone does not transform an object. A physical act of preparation is required, an actual reshaping of the item into a seat, and only then does it enter the category of things that receive tum'at midras.
From intention the mishnah moves to a question of measurement: "kisei shel katan", a chair belonging to a young child, "sheyesh lo raglayim", one that is built up on legs. Such a chair is ruled "tamei", susceptible, even in a case where the seat does not rise a tefach above the floor ("she'ein bo govah tefach"). A grown person would get little use out of a bench that low, but adult comfort is not the yardstick here. The chair was made for children, and for the child who uses it, it functions perfectly well as a place to sit. Being a real seat for its user, it belongs to the category of items that take on tum'at midras.
The mishnah closes with a garment: "chaluk shel katan", the shirt of a little child. Rabbi Eliezer's position is "kol shehu", that any size whatsoever is enough. Even a minute wrap sewn for a newborn already accomplishes exactly what it was made for, clothing a baby, and that suffices. Clothing as a class is open to tum'at midras, since people do at times lean or sit upon their garments, and to Rabbi Eliezer an infant's shirt is a garment in the full sense, however little cloth it contains.
The chachamim take the opposite view: "vachachamim omrim", they require "ad sheyehei bo kashi'ur", that the shirt hold the standard measure, and they add "venimdad kaful", that the measuring be done while it is folded over. The shiur is the ordinary one for the material involved, cloth or sack and similar substances, three by three or four by four. Since a shirt encloses the body, part of it lying in front and part behind, measuring it folded means that each of the two sides must contain that minimum by itself. Behind their ruling lies an assessment of how the item is actually used: a newborn's shirt serves for only a brief stretch of time before the baby outgrows it, and use that fleeting does not establish the cloth as a true utensil that a person would sit or recline upon. On their view, then, the shirt remains tahor until it reaches the required dimensions.
Read together, the three cases of the mishnah revolve around one question: what makes something a seat or a garment in the eyes of the halachah. Rabbi Akiva looks to the owner's decision, the chachamim look for a physical change or a standard measure, and in between stands the child's chair, which needs neither intention nor extra size, because it already does exactly what a chair is meant to do for the one who uses it.