Halakha is unique in its precision and its focus on even the tiniest details. This precision elevates all our activities and gives them spiritual meaning and value. It should come as no surprise, then, that the Torah prohibition of cooking applies not only to large quantities of food, but even to the smallest drops of water. Because of this, some maintain that one may not pour hot water from an urn into a cup that has droplets of cold water inside, because the hot water will cook the cold water. Therefore, one must thoroughly dry out the cup, and only afterward may one pour hot water into it (Igrot Moshe, OĤ 1:93; Minĥat Yitzĥak 9:30; SSK 1:52).
In contrast, many poskim maintain that it is unnecessary to dry the cup, because this pouring is not a normal way of cooking. Also it is doubtful whether in fact the drops will cook. Even if they do, the person does not intend for them to cook, nor does he know if they have cooked. His intent when he pours the water has nothing to do with these droplets. The halakha follows this approach (Tzitz Eliezer 13:40; Shevet Ha-Levi 7:42; Yabi’a Omer 4:33).
. It is true that during Temple times, in order to be liable for a sin offering for cooking on Shabbat, one must have unknowingly cooked a quantity of water that would be at least enough to rinse a small limb (MT 9:1). Nevertheless, cooking even one drop is still prohibited by Torah law. This is at the heart of the opinion of those who are stringent in our case. Even though the droplets of moisture are in a kli sheni, nevertheless the hot water was poured from a kli rishon, and the consensus is that this cooks the outer layer of the food.On the other hand, there are many reasons to be lenient in this case. Since these drops are not being cooked in the normal way, the prohibition is rabbinic. Since one does not benefit from the results of this cooking, it is considered a psik reisha de-lo niĥa lei, which some permit in the case of a rabbinic prohibition. Furthermore, this might not even be a psik reisha, since the poured water might not reach the drops directly (see above ch. 9 n. 2). Additionally, according to Ĥakham Tzvi §86, since the reason halakha forbids ĥatzi shi’ur (violating a prohibition below the threshold of punishment) is that the person has subjectively deemed it important, when he does not intend the prohibited result there is no violation even if it is a psik reisha. Tzitz Eliezer 13:40 states similarly that these drops have no importance. Moreover, we can add to the equation the position of Rashbam that irui from a kli rishon does not cook. Also, if the moisture in the cup is from hot water that has cooled down, some permit this, applying the principle of ein bishul aĥar bishul here (see sections 5-6 above). In practice, since even those who prohibit this admit that it is a rabbinic prohibition (since the one pouring the water is not interested in cooking the drops), the halakha follows those who are lenient. Based on most of the reasons presented here, there is no need to dry off a spoon that one wants to use to remove food from a kli rishon (Shevitat Ha-Shabbat, Introduction to Mevashel 19; Tzitz Eliezer 13:40).
Some are concerned about allowing the use of an electric urn that has a tube running down the side that displays how much water is inside. The fear is that as the water in the urn is used up, some of the water from the tube enters the urn; if this water was cold it will be cooked by the hot water in the urn. But many are lenient about this. First, the person does not intend for this to happen. Additionally, it may be that the water in the tube was already yad soledet bo. Even if this is not the case, it might have been cooked to yad soledet bo before Shabbat. Then, according to those who maintain that ein bishul aĥar bishul applies to liquids (see sections 5-6), one may reheat it on Shabbat (Az Nidberu 9:14; Yeĥaveh Da’at 6:21).