One of the thirty-nine prohibited labors on Shabbat is cooking. This prohibition is derived from the baking that was performed for the Tabernacle, and it includes all kinds of preparation of food for consumption by means of heat: cooking, frying, roasting, and the like.

The labor that is prohibited on Shabbat is not only cooking in its classical sense, meaning the initial placement of a pot containing raw materials on a fire. Rather, any heating of foods that are not cooked and that contain liquid, to a certain temperature is considered cooking. The halakha defines the degree of heat that is considered cooking by means of the point at which one’s hand spontaneously recoils from the item’s heat [yad soledet bo]. In modern terminology, this is a temperature of roughly 45° C (113° F) and above.

Even if the person intends to remove the food from the source of heat before it gets cooked, it is prohibited on Shabbat to place it on top or inside a source of heat that could eventually bring it to the prohibited temperature.

With regard to the prohibition of cooking on Shabbat, the halakha distinguishes among three basic types of vessels, with a different law applying to each: a primary vessel [keli rishon], a secondary vessel [keli sheni], and a tertiary vessel [keli shelishi]. The primary vessel is one that is placed on the heat source itself. Thus, a vessel that is positioned on a flame is a primary vessel. Likewise, a vessel that is placed on a Shabbat hot plate is also a primary vessel, as is an electric water heater. A primary vessel retains this status even after it has been removed from the heat source, until it has cooled off. Adding food to a primary vessel is prohibited on Shabbat as a form of cooking. Later, we will explain this in greater depth, and show how there are cases and situations in which one can nevertheless derive benefit from the heat of a primary vessel.

A “secondary vessel,” as its name implies, is one to which food was transferred after it was cooked in a primary vessel. The sides of the second vessel begin as cold, and although the hot food that was placed into it warms them, the halakhot regarding this vessel are more lenient than those of a primary vessel, as will be explained below.

A tertiary vessel is one to which the food was transferred from a secondary vessel. One can usually assume that the heat of this vessel and its contents have cooled to such an extent that they will no longer cook. Nevertheless, there are some foods that maintain a particularly high temperature even when they are in a tertiary vessel, and therefore it is prohibited to place ingredients inside them that have not yet been cooked. This too will be detailed below.