Using a set Keli Sheiyni-Must one dry his cup/plate prior to pouring into it from a Keli Rishon?
If the liquid in the cup/plate is liquid that has never been previously cooked: Prior to pouring Keli Rishon water into a cup or other utensil one must verify that that the cup/utensil is dry. Some Poskim however rule it suffices to merely shake out the liquid from the cup/plate as opposed to fully drying it. Other Poskim however rule that one may not be lenient in this matter and the cup must be completely dry. Practically, although one who is lenient has upon whom to rely, one should try to be stringent whenever possible, and use a dry cup/plate, or dry it in a permitted method. [When drying one’s cup one may not use a cloth due to chance that one may come to squeeze liquid from it. Rather one should use his finger to dry it.]
If the liquid in the cup is from liquid that has been previously cooked: If the liquid in the cup/plate has been previously cooked, as commonly occurs when pouring a second serving onto a plate/bowl or cup, then even if the leftover liquid has fully cooled down, one may pour from the Keli Rishon onto it. [Thus, one may designate a set cup to be used as a Keli Sheiyni to pour from it onto a Keli Shelishi of tea/coffee and is not required to use a new Keli Sheiyni for every time one pours. One does not have to dry the set Keli Sheiyni prior to each use, even if the liquid has already cooled off.]
 Shabbos Kehalacha Vol. 1 p. 116; Piskeiy Teshuvos 318/21 [old] 51 [new]
 M”A 318/35; Tosafus Shabbos 42a that pouring on a minute amount of cold water is forbidden; Tzemach Tzedek Mishnayos Shabbos 3/5, brought in Ketzos Hashulchan 124/12 footnote 31; P”M 318 A”A 35; Gr”a; Chayeh Adam; Tehila Ledavid 318/28; M”B 318/78 and 85; Biur Halacha 318 ”Vehu Shelo”; Kaf Hachaim 318/130; Ketzos Hashulchan 124/11-12; Igros Moshe 1/93; 4/74; Minchas Yitzchak 9/30-31; Beir Moshe 6/110; ; Az Nidbaru 2/20; Teshuvos Vehanhagos 1/207; Piskeiy Teshuvos 318/51
Other opinions: Some Poskim rule that pouring from a Keli Rishon onto a minute amount of cold is permitted. [Admur 318/20; Taz 318/18; Tosafus Pesachim 40b; Tur 318; Rashba, Ran and Rif on Shabbos 20a; Elya Raba Y.D. 68; Peri Megadim Yoreh Deah 68; Iglei Tal Ofeh 29; Tzemach Tzedek Mishnayos Shabbos 3/5, brought in Ketzos Hashulchan 124/12 footnote 31, and Tehila Ledavid 318, in understanding of Michaber/Admur]
 Har Tzevi Tal Harim Mivashel 1; Sheivet Haleivi 7/42-2; Mishneh Halachos 6/67; Tzitz Eliezer 13/40; Divrei Yatziv 156; Vayan Yosef 1/93; Ateres Moshe 1/86; Yabia Omer 4/33; Mishneh Halachos 6/67; Shevet Hakehasi 2/146; Maor Hashabbos 8 footnote 103 and 4/33 in name of Rav Elyashiv; See Piskeiy Teshuvos 318/51 footnote 461
 The reason: As they rule the remaining drops are considered irrelevant, and it is a Pesik Reishei Delo Nicha Lei, as well as many Poskim ibid rule it is even initially permitted. [Poskim ibid]
 Igros Moshe 1/93; 4/74; Minchas Yitzchak 9/30-31; Beir Moshe 6/110; Az Nidbaru 2/20; Teshuvos Vehanhagos 1/207; See Piskeiy Teshuvos 318/51
 See Admur 302/23
 Tzemach Tzedek Mishnayos Shabbos 3, brought in Ketzos Hashulchan ibid footnote 31; Ketzos Hashulchan 124/12; Shevisas Hashabbos Hakdama Mivashel 19; Or Letziyon 2/30-10; Beis Meir brought in Biur Halacha “Vehu” that those that are lenient do not need to be protested; Igros Moshe 4/74 Bishul 18; Piskeiy Teshuvos 318/51
Other opinions: Some Poskim rule it is forbidden to pour from a Klei Rishon even onto precooked water. [Implication of M”B 318/39 [regarding the process of making tea on Shabbos] that it is forbidden to pour from a Keli Rishon even on precooked liquids. See Biur Halacha “Vehu” in name of Beis Meir that one does not need to protest against one who does so, although each person should be Machmir; See Igros Moshe 4/74 who is only lenient by a few drops]
 The reason: As reheating a cold precooked liquid is itself a dispute, and there are Poskim who are completely lenient even with uncooked liquids, therefore by precooked thus by a double dispute one may be lenient even initially. [Tzemach Tzedek Mishnayos Shabbos 3, brought in Ketzos Hashulchan 124/12 footnote 31; Piskeiy Teshuvos 318/51 footnote 462] Now, although there he concludes that nevertheless it is proper to be stringent, here since there are Poskim who are completely lenient even with uncooked drops, therefore one may be lenient even initially. [See Shabbos Kehalacha ibid] See also Igros Moshe 4/74 that is lenient in this matter by a cup with leftover drops, although in that case it is due to other reasons as well and hence in a case of a lot of liquid he is stringent.