Parah, Chapter 6, Mishnah 2. This perek deals with the act of kiddush, the sanctifying of the spring water by placing into it the ashes of the parah adumah, and with the mishaps that can occur in the middle of that act. Our Mishnah presents two such mishaps, and in each one the question is the same: after the ashes have already come into contact with the water, can a person still pick them up and use them for a fresh kiddush?
The First Case: Ashes Floating on the Surface
The Mishnah begins: "Hakiddush tzaf al pnei hamayim", the sanctifying ashes floated on the surface of the water rather than sinking into it and mixing with it. Rabbi Meir and Rabbi Shimon rule "notel u'mekadesh", he may lift those ashes off the water and use them to sanctify. In their view, ashes that merely rested on top of the water have not yet accomplished anything, and they retain their status as ashes available for kiddush.
The Sages take the opposite view, and they express it as a broad rule: "kol shenaga bamayim", whatever has come into contact with the water, "ein mekaddshin bo", can no longer serve as the means of sanctifying. For them the touch itself is conclusive, and those floating ashes have finished their role.
The Second Case: Ashes Found at the Bottom
Now the second mishap: "zalaf es hamayim", he tipped the water out of the utensil, "v'nimtza kiddush milmatah", and there lay the ashes at the bottom, having sunk down through the water without blending into it. Rabbi Meir and Rabbi Shimon again allow a second use, "menagev u'mekadesh": he wipes them dry and performs kiddush with them. The Sages, for their part, apply the very same rule as before, "kol shenaga bamayim", once the water has touched it, "ein mekaddshin bo", it is no longer suitable for sanctifying.
The Halacha
In both of these disputes the halacha follows the Chachamim. Even if the ashes were dried off first, the fact that they once made contact with the water is decisive: with that contact the mitzvah of kiddush was already carried out through them, and ashes that have already served for a kiddush are no longer fit to be used for sanctifying water again.
The lesson of this Mishnah, then, is that the moment of contact between the ashes and the water is the halachically significant moment. Whether the ashes hovered above the water or settled below it, they have played their part, and a person may not reclaim them for a second act of sanctification.