Rabbis Cook by the Book
Everyone knows what cooking is. You apply heat to a food in order to make it softer and more palatable for consumption.
Halacha is concerned about cooking since we are not permitted to cook meat with milk. Although by Rabbinic decree we do not combine meat and milk in any fashion, not even consuming milk for some hours after having consumed meat, the Torah prohibition applies exclusively to cooking meat with milk. Cooking must also be defined re the Torah prohibition against cooking during Shabbos. This requires great precision, human life depends upon it, since it relates to capitol offences, matters of life and death.
Halacha does not define cooking through temperatures and not even through the changes to the product. Cooking is defined by Halacha as achieving various changes to foods through applying certain types of heat. These heat types are most well known as Keli Rishon, the First Pot; and Keli Sheni, the Second Pot. The First Pot is that which is being or has been, heated on a fire, it has the power to cook. The Second Pot into which the contents of the First Pot have been decanted, does not cook. This is known as a rule - Keli Rishon MeVashel, Keli Sheni Eino MeVashel.
This is quite sensible, one thinks, since these guidelines were developed at a time when thermometers had not yet been invented. Measuring temperatures was accomplished through evaluating the colour of the fire or by touch. From this perspective, establishing a rule of thumb such as classifying the pot and contents that was heated on the fire and differentiating it from the Second Pot appears to be a pretty good guide.
But there is a problem. A Second Pot may well be far hotter than a First Pot, yet the same food added to the hotter pot will not be "cooked" yet it will be "cooked" when added to the First Pot. And we would execute the cooker who added to the cooler First Pot but not the coker who added to the hotter Second Pot.
Tosafos in Avodah Zara propose that a small pot immersed in the boiling contents of a larger pot, will never attain the status of a Keli Rishon; even when the contents of the smaller pot are boiling. Although we do not rule accordingly, we nevertheless do not dismiss this ruling. Therefore in any case where a Keli Rishon is required, using anything other than what was directly on the fire, is unacceptable.
What is the foundation upon which these remarkable arguments are founded?
It would appear that unless there has been direct contact with a heat source, there can be no Keli Rishon status. Perhaps the consideration is that although the contents of the pot reach a maximum temperature after which more heat only causes the pot to boil faster, the pot itself will reach a far higher temperature. We may now define a Keli Rishon as a pot that continues to add heat to its contents even after it has been removed from the fire. The pot itself is a heat source for what is contained within the pot.
When a small pot is immersed in a larger boiling pot, the immersed pot never gets hotter than the contents of the larger pot. For this reason it will not attain the status of a Keli Rishon.
The Torah's paradigm differs significantly from what we picture to define cooking. Another illustration that highlights this is the debate whether a hot food dropping onto a cold food constitutes cooking but not the reverse, if a cold food drops onto a hot food. According to the known Laws of Thermodynamics, there is no conceivable difference. However, our Sages debate this in the Gemara. And we Pasken, we rule, that the reaction is determined by the lower component. When the lower is hot we have cooking, when the lower is cold we do not have cooking.