פירוש על כלים 28:1

שלש על שלש שנתנה בכדור – a cloth of three fingerbreadths by three-fingerbreadths that was placed within a ball, PLOTA in the foreign language.

[A piece of cloth] three [fingerbreadths] square that was stuffed into a ball or was itself made into a ball is clean.
But [a piece of cloth] three [handbreadths] square that was stuffed into a ball remains unclean.
If he made it into a ball itself, it is clean because the sewing reduces its size.
Section one: By making this small piece of cloth either into a ball or using it as stuffing for a ball, the cloth loses its importance and it becomes clean from impurity. Note that this does not mean that the ball is necessarily clean. It just means that the piece of cloth that used to be unclean has shed its impurity.
Section two: In contrast, if one takes a larger piece of cloth and puts it into a ball it is still susceptible to impurity because it is still significant. However, if he sews the cloth into a ball itself, it still loses its impurity because it loses its size. A ball that is made of a less than three handbreadths piece of cloth is not large enough to be susceptible to impurity.

או שעשאה כדור – that he took a cloth and wrapped it up round like a kind of ball, like the young children do, and sewed it.

טהורה – that it became voided when it it was placed in a ball, and all the more so, when he made a ball of it by itself that it became voided through sewing.

אבל שלשה על שלשה (but three – handbreadths—by three – handbreadths) – it was not voiced through its being placed within the ball, because it is preserved there. But if he made of it a ball by itself, it is ritually pure, because the sewing diminishes it, and it is pure from treading/Midras uncleanness. But it is impure in all of the other defilements according to the law of all three-handbreadths by three-handbreadth objects that were diminished.