Keilim, Chapter 28, Mishnah 1. This mishnah returns to a subject that runs through the tractate: the minimum size a piece of fabric must have before it counts as something that can contract tum'ah at all, and what happens when a scrap of cloth is put to a new use.
Two measurements need to be clear before the mishnah can be understood. A garment or piece of cloth that is to receive tum'at midras, the impurity that comes from being sat or trodden upon by a zav or the like, must measure at least three tefachim by three tefachim. For every other form of impurity, a much smaller piece suffices: three etzba'ot by three etzba'ot, that is, three fingerbreadths square.
Cloth stuffed into a ball
The mishnah opens with a small scrap: "shalosh al shalosh", a piece of fabric of three fingerbreadths by three fingerbreadths, "shenetanah bekadur", which was pushed into a ball. Children of that era played with a ball built from an outer shell that was packed tight with filling, hair or assorted odds and ends of material. Our little scrap has been put to precisely that use, serving as padding inside the shell.
A second scenario follows immediately. Instead of functioning as filling, the scrap may itself be converted into the ball: "o she'asa'ah kadur", or one fashioned it into a ball, "bifnei atzmah", standing on its own. It is wound together, sewn shut and given a shape that lets it serve from here on as nothing but a ball.
In both of these cases the ruling is "tehorah": the cloth is not susceptible to impurity. A piece this small forfeits whatever standing it had as a garment once it is turned into padding or bunched up into a ball. It is no longer a cloth; it is filler or a toy.
A larger cloth stuffed into a ball
The mishnah now takes up a sizeable cloth: "aval sheloshah al sheloshah", a fabric measuring three tefachim by three tefachim, "shenetano bekadur, tamei", which was used to pack out a ball, and it is susceptible. Every category of impurity applies to it, tum'at midras included. Fabric of these dimensions is far too significant to be downgraded merely because its owner used it as filling; it retains its standing as a genuine cloth, and its shiur remains intact.
The same cloth made into a ball
The concluding case reads "asao kadur bifnei atzmo": that same cloth of three tefachim by three tefachim was not used as filling but was itself fashioned into the ball, wound up and sewn into shape. Here the ruling is "tahor", and the mishnah supplies the reason: "mipnei shehatefer mem'ato", because the seam diminishes it. Nothing has happened to the cloth's importance; what has changed is its dimensions. Gathering the fabric with stitches contracts it, so that it no longer contains a full three tefachim by three tefachim, the measure without which tum'at midras cannot apply.
The mishnah thus teaches us to look at two things whenever a cloth is put to a new use: whether the piece is large enough to retain its standing as a garment despite the new function, and whether the very act of refashioning it has cut it down below the measurement its impurity requires.