Mayim Shenisgalu-Liquids that were left uncovered:[1]

The letter of the law: Liquids that were left uncovered [without supervision[2], for even a very short amount of time[3]] were forbidden by the Sages to be consumed [or used[4]] due to worry that perhaps a snake entered venom into the liquid [and one will come to drink it and die].[5] [This prohibition applies to all liquids, including wine.[6] The prohibition applies whether the liquid was left uncovered during the day, or during the night.[7]]

The custom today: Today, being that snakes are no longer commonly found in inhabited area, it is permitted to drink liquids that remained uncovered.[8] It is thus no longer accustomed to be careful in this matter.[9] [However, some Poskim[10] rule that even today one is to avoid drinking liquids that were left uncovered.[11] Certainly in those areas in which poisonous snakes are commonly found, it is forbidden from the letter of the law even today to drink liquids that were left uncovered.[12] Due to this, some Poskim[13] rule one is to be stringent in this matter in Jerusalem, and Eretz Yisrael, and any area that they are slightly found. Each person is to verify how common it is to find poisonous snakes in the homes of one’s area, and act accordingly. In any event, in all cases, it is proper to beware from leaving foods and liquids uncovered due to the insects that can enter them and cause one to stumble in eating them.[14]]

Wine that was left uncovered: Wine that was left uncovered for some time is invalid to be used for Kiddush, as explained in 272/1.

______________________________________________________

^[1] Michaber Y.D. 116:1; O.C. 272:1; Admur O.C. 272:1; Tosefta Terumos 7:13-19; Avoda Zara 30a-b; Chulin 9b

[2] Tur 116 that a live person who is guarding the liquid will prevent a snake from coming out to drink from it.

[3] Tur 116; Aruch Hashulchan 116:3

Regarding the amount of time that it must be left revealed to be prohibited: Seemingly this includes even a short amount of time, so long as the snake has enough time to reach it and place venom into it.

[4] Tur and Beis Yosef 116 rule the water is not to be spilled in a public area and is not to be given to animals or birds and is not to be used to wash one’s floor.

[5] Michaber Y.D. ibid

^[6] Admur ibid; Michaber O.C. ibid; Some Poskim rule that this includes honey and even ground garlic. [Perisha 116:2]

^[7] Beis Lechem Yehuda 116:1; Poskim in Kaf Hachaim 116:7; See Hakashrus p. 431 footnote 67; So is proven from fact that the Poskim make no differentiation in this matter.

[8] Michaber Y.D. ibid

The reason for this allowance: As the entire prohibition of the Sages was due to the snakes. [Taz 116:1] Now, although, in general, when the Sages make a decree the decree remains in place, even if the reason behind the decree is no longer applicable, until a larger and more scholarly Beis Din nullifies it. Nevertheless, this only applies in decrees that were made due to a reason but not made contingent to the reason, such as the decree not to do Melacha on Erev Pesach due to the Pesach sacrifice. However, in this scenario, the Sages explicitly made the decree contingent to its applicable reason, and hence in a scenario that the reason is not applicable, the Sages initially never decreed against drinking it. [See Tosafos Avoda Zara 35a that at times the decree is not applicable] Water that was left revealed in an area without snakes, is similar to revealed water that was supervised to make sure no snakes entered, in which case it is certainly permitted. [Mateh Yehonason 116:1, brought in Otzer Mefarshim]

[9] Admur ibid; Michaber O.C. ibid; Tur 116; Taz ibid; Levush 116; Peri Toar 116:2; Peri Chadash 116:1; M”B 160:23; Rebbe in Likkutei Sichos 23:35; All Poskim in later footnotes who only require in area that snakes are commonly found

^[10] Shlah p. 79 “One who guards his soul is to distance himself from this” [brought in Elya Raba 170:24; P”M 170 M”Z 10; Piskeiy Teshuvah 116:1; Aruch Hashulchan 116:2; Likkutei Sichos 23:35 footnote 32] Custom of the Gr”a who was very careful in this [Maaseh Rav 95, brought in Pischeiy Teshuvah 116:1]; Peas Sadcha 86; The M”B 160:23 holds that there is no need today to be stringent in drinking revealed waters, although there are those who are stringent; Kitzur SH”A 33:5 [To note that in the Kitzur SH”A with the rulings of Admur brought by Rav Bistritzky no mention is made in that Halacha that Admur argues on the Kitzur in that regard.] Tosafos Yerushalayim 116 [brought in Hakashrus ibid] writes based on the Mishneh in Derech Eretz 11 that the nighttime is more severe than the day in this regard and hence all waters that were left revealed, even at night are not to be drunk.

^[11] The reason: Seemingly this is due to Ruach Raah. [Peias Hasadeh ibid]

[12] Peri Toar 116:2; Peri Chadash 116:1; Yeshuos Yaakov 116:2; Birkeiy Yosef in Shiyurei Bracha 116:3; Mor Uketzia 170;  Aruch Hashulchan 116:2; Kaf Hachaim 116:6

[13] Peri Chadash 116:1 “In all areas that they are even slightly found, such as Yerushalayim, it is proper for every G-d fearing Jew to avoid drinking it”; Birkeiy Yosef ibid that so is the custom today in all Eretz Yisrael

[14] Zivcheiy Tzedek 116:2; Kaf Hachaim 116:9

^[15] See Hakashrus p. 432; Piskeiy Teshuvos 4:12

[16] Shalmei Tzibur brought in Shaareiy Teshuvah 4:7; Pischeiy Teshuvah Yoreh Deah 116:1; Peri Toar Yoreh Deah 116:2; Birkeiy Yosef 116:4; So is also implied from M”B 160:23; Admur 272:1 rules we are no longer careful regarding revealed waters as snakes are no longer commonly found.

[17] Shalmei Tzibur ibid, as the stringency of not drinking from revealed waters in areas with snakes only applies to drinking and not washing. [Shaareiy Teshuvah ibid; M”B 160:23]; See however Poskim in next footnote that one is to be stringent in those areas that snakes are common, even regarding washing.

[18] Rav Sheptal brought in Shaareiy Teshuva ibid; Seder Hayom: “Every G-d fearing Jew needs to prepare for himself prior to going to sleep two vessels, one that contains water and one that is empty, into which he will pour the water upon awakening. One is to place the water filled vessel into the empty vessel, covered and clean.”; Chesed Lealafim 4:10; Kaf Hachaim Falagi 8:11; Kaf Hachaim 116:8 and Artzos Hachaim 4:1 that one is to be stringent in those areas that snakes are common [brought in Hakashrus 18:19]

[19] Conclusion of Shaareiy Teshuvah ibid

[20] See Migdal Oz story 114 from which it is proven that it was not an acceptable Chabad custom to cover the waters. This is seen from the fact the servant of the Tzemach Tzedek could not understand why he covered the water, as well as from the fact one Chassid complained to the Rebbe Rashab about the stringencies of another Chassid who followed this ruling of covering the water. Nevertheless, as is evident from the story, this was the custom of the Tzemach Tzedek. However, from his response it is understood that this custom is not meant as a directive for the public.

^[21]  Levush 160:5; Aruch Hashulchan 160:1; M”B 160:23 [It is implied from there that he rules even regarding washing hands upon awakening there is no need to be stringent]; See however Kaf Hachaim 116:8 that one is to be stringent in those areas that snakes are common.

[22] Tur and Beis Yosef 116; Hakashrus 18:15

[23] Kaf Hachaim 272:9; Hakashrus 18:17

[24] Peri Chadash 116:3; Birkeiy Yosef Shiyurei Bracha 116:5