One of the 39 prohibited activities on the Sabbath is bishul, or "cooking." However, bishul is not an exact equivalent of "cooking." The Hebrew term bishul as it relates to Shabbat is the "use of heat to alter the quality of an item," and this applies whether the heat is applied through baking, boiling, frying, roasting and most other types of cooking. The prohibition of bishul applies to all types of food and drink, even to foods and drinks which are edible when raw or cold.
Heat sources
The prohibited activity of bishul is separate and distinct from that of havarah. Performing bishul with a pre-existing flame is forbidden on Shabbat The prohibition of bishul, however, is not limited to the use of fire as a heat source; it is forbidden to perform bishul with any source of heat, whether it be an actual flame, or an electric stove/range, a hot plate, an urn or a microwave oven. Moreover, placing food into a kli rishon may constitute bishul in certain instances. However using heat from the sun to cook is allowed on shabbat.
Reheating foods
While it is prohibited in most instances to initially heat a food item to the temperature of yad soledes bo, foods that have already been fully cooked may sometimes be reheated. In terms of reheating, a distinction is made between dry foods and liquids. Dry food that has been completely cooked is no longer subject to the prohibition of bishul; this is based on the principle of ain bishul achar bishul. Thus, a completely cooked, dry food item, such as a piece of chicken or potato kugel, may be reheated once it has been fully cooked. However, there is a great dispute as to whether this rule applies to liquids: Maimonides, the Rashba and the Ran assert that liquids are in fact no different from solid dry foods, whereas Rashi, the Rosh and Rabbeinu Yonah assert that this rule does not apply and reheating of liquids is forbidden, applying the principle of yeish bishul achar bishul to liquids. This prohibition of reheating liquids only applies when the liquid has completely cooled. If the liquid has only partially cooled and still retains enough heat to be enjoyed as the warm liquid as it was intended to be, it may be reheated. Kli rishon, literally the first utensil, refers to a utensil that is used for cooking, baking or roasting food or liquid, and contains that hot food or liquid. When hot food or liquid is transferred from the kli rishon into a second utensil, this utensil is called a kli sheni. A kli shlishi is the third utensil into which hot food or liquid is transferred.
Hot beverages
The problem of preparing hot beverages on Shabbat revolves around the temperature of the water. If the water is hot enough to cook the tea leaves, it would constitute malacha. Pouring straight from an urn would cause the cooking of the substance. For a solid substance the outer and concrete layer will definitely be cooked, which would be malacha. For a liquid there is no concrete layer, and therefore no specific part that is being cooked. Therefore, it is ruled that a liquid is not considered cooked if it is not yad soledet bo or. A kos sheini can be used to bypass this problem. Using a Kos sheini is acceptable because when the liquid is poured some of the heat is transferred into the atmosphere, and therefore the liquid loses some heat. Most people hold that this will not cause enough heat to be emitted and therefore the tea leaves will still be cooked. To lower the temperature of the water further people rule that a klei shlishi must be used. Once again in the pouring process more heat is emitted and therefore some people hold that the tea will not be cooked and it is therefore permissible to make tea with this water. However, many other authorities hold that tea leaves fall under the category of items which cook easily, even in the diminished heat of a kos shelishi. Consequently, those who are most scrupulous in their observance will prepare a concentrated tea extract before the Sabbath; as a liquid, water from a kos sheini can be added to the extract to heat it.
