Some people like to drink hot water with a lemon flavor, which they prepare by dropping or squeezing a piece of lemon into a cup of hot water. Under what conditions would this be permissible on Shabbat?
With regard to the laws of Bishul (cooking) on Shabbat, Halacha differentiates between a "Keli Rishon" – the original utensil in which an item was cooked, such as a hot water urn – and a "Keli Sheni" – the utensil into which the item was poured. Liquid in the original utensil has the capacity to "cook" raw food items placed inside it, even after the utensil has been removed from the fire. However, if one would pour the liquid from its original utensil into a different utensil, then it no longer has the capacity to cook, as it has been cooled by the walls of the second utensil. Thus, for example, when a person pours some hot water from an urn into a cup, he may, generally speaking, place raw food items into the cup on Shabbat.
There are, however, certain "sensitive" food items – known as "Kaleh Ha’bishul" – that are deemed capable of being cooked even in a "Keli Sheni." Such foods may not be placed into hot liquid in a "Keli Sheni." Instead, one must transfer the liquid from that utensil into a third utensil – a "Keli Shelishi" – and only then place the raw food into the water.
The Halachic authorities debate the question of whether or not lemon falls into this category of "Kaleh Ha’bishul." According to some opinions, the sharp and tangy nature of a lemon render it more susceptible to cooking, and therefore one may not place a lemon into a cup that one filled with hot water directly from an urn. He must instead transfer the water to yet another utensil and then place the lemon. The Hazon Ish (Rabbi Abraham Yeshaya Karelitz, 1879-1954), however, ruled leniently on this issue, and maintained that one may place a lemon directly into a "Keli Sheni."
As for the final Halacha, Hacham Ovadia Yosef accepted the lenient ruling, but added that it is preferable to act stringently. Since there is little effort entailed in transferring the hot water to a third utensil, it is proper to do so in the interest of satisfying all opinions. (One may also place the lemon in the third utensil and then pour the hot water on top of the lemon to produce the lemon-flavored water.)
Summary: Strictly speaking, one may place a lemon into a cup of hot water that had been filled directly from an urn. It is proper, however, to first transfer the water to another cup and then add the lemon.
