Parah, Chapter 6, Mishnah 3. This perek deals with the act of kiddush, the sanctifying of spring water with the ashes of the parah adumah, and with the various mishaps and complications that can arise while it is being performed. Our Mishnah looks at a case where the kiddush is done not in a single vessel but in a large trough, with a smaller vessel sitting inside it, and it then turns to a case where something absorbent is lying in the water.
A Jug Inside the Trough
Our Mishnah opens with "Hamekadesh bashoket", one who carries out the kiddush in a trough, "v'hatzfi b'socha", at a time when a jug is resting inside that trough. Then comes the ruling: "af al pi", even though, "shepiv tzar kolshehu", the jug's opening is narrow, however slight that opening happens to be, nevertheless "hamayim sheb'socho mekudashin", whatever water sits within the jug becomes sanctified no less than the water surrounding it. The reason is continuity: the two bodies of water meet through that opening and are reckoned as one, so the ashes placed into the trough take effect on the jug's contents as well.
A Sponge in the Water
Now the Mishnah substitutes a different object: "Im hayah sfog", should the item lying in the trough be a sponge rather than a jug, then "hamayim sheb'socho psulin", the moisture soaked up inside it is disqualified. Liquid held within a sponge does not carry the status of water resting in a utensil, and water of that sort lies beyond the reach of the kiddush.
The Mishnah therefore asks, "Keitzad ya'aseh", what is a person to do in such a situation? The answer: "yizalef ad shehu magi'a lisfog", he pours off the water gradually until he reaches the sponge, taking the fit water for use and stopping before he comes to the sponge itself.
Touching the Sponge
A caution follows. "Naga basfog", should he come into contact with the sponge, then "af al pi", even in a case where, "shehamayim tzafin al gabav", water is still riding on the surface above it, that remainder too, "kolshehein psulin", in whatever amount it happens to be, is ruled unfit. Why? The contact presses liquid out of the sponge, and that liquid mingles with the water which had until then been valid; those invalid drops ruin the entire quantity.
The Mishnah thus draws a clear line between a vessel and a sponge. A jug inside the trough, however narrow its opening, holds water that joins the kiddush and is sanctified. A sponge holds water that can never be sanctified, and a single careless touch turns the good water above it unfit as well.