We now move on to Mishnah 2 in the twenty-second chapter of Maseches Shabbos. This Mishnah is a pause and a return to the laws of cooking on Shabbos, and in it we will learn the laws of soaking and rinsing in hot water.
Something that was in hot water before Shabbos:
The Mishnah opens: "Kol davar shebah bechamin me'erev Shabbos" - anything that came into hot water before Shabbos. The Rishonim disagree about how to define this:
Rashi: This refers to something that was actually cooked before Shabbos.
Tosafos: It must have been cooked fully, that is, completely cooked.
Such an item may be soaked in hot water on Shabbos, and even in a kli rishon, provided that it is not standing on the fire. Even if the vessel was just now removed from the fire, one may place the item into it.
Something that was not in hot water before Shabbos:
Even something that was not cooked before Shabbos may have hot water poured over it on Shabbos. This permission is explained as referring only to a kli sheni, since the halachah is that pouring from a kli rishon cooks at least kedei klipah - the thin outer layer of the food beneath is cooked. Therefore one may not pour from a kli rishon onto something that was not cooked before Shabbos, while pouring from a kli sheni is permitted.
The exceptions:
The Mishnah qualifies that there are certain items onto which one may not pour hot water even from a kli sheni, since for them a small amount of hot water is enough, and even water that is not especially hot, to have an effect:
Hamaliach hayashan: a salted item, and in particular a fish that was salted and a full year has passed since it was salted.
Small salted fish.
Kolyas ha'ispanin: a particular type of Spanish fish.
The reason for this, in the words of the Mishnah: "shehadachasan zo hi gemar melachtan" - that their rinsing is the completion of their preparation. Pouring the hot water over them is what makes them fit to eat, and this is literally the final act of cooking for them. For that reason it is forbidden on Shabbos, even though with other foods this would not be considered cooking.
In summary: In this Mishnah we learned that something that was in hot water before Shabbos - according to Rashi, that it was cooked, and according to Tosafos, that it was cooked fully - may be soaked in hot water on Shabbos, even in a kli rishon that is not on the fire. Something that was not in hot water before Shabbos may be rinsed only from a kli sheni, since pouring from a kli rishon cooks kedei klipah. Excluded from this rule are the old salted fish, small salted fish and kolyas ha'ispanin, whose rinsing is the completion of their preparation and is therefore forbidden.