Mayim Shenisgalu-Liquids that were left uncovered:
The letter of the law: Liquids that were left uncovered [without supervision for even a very short amount of time] were forbidden by the Sages to be consumed [or used] due to worry that perhaps a snake entered venom into the liquid [and one will come to drink it and die]. [This prohibition applies to all liquids, including wine. The prohibition applies whether the liquid was left uncovered during the day or during the night.]
The custom today: Today that snakes are no longer commonly found amongst us it is permitted to drink liquids that remained uncovered. It is thus no longer accustomed to be careful in this matter. [However, some Poskim rule that even today one is to avoid drinking liquids that were left uncovered. 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. Due to this, some Poskim rule one is to be stringent in this matter in Jerusalem, and Eretz Yisrael, and any area that they are slightly found. 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.]
It is permitted to drink liquids that were left uncovered unless one is in an area that poisonous snakes are commonly found. Some are accustomed to be stringent against drinking uncovered liquids in all cases. In any event, it is proper to beware to cover the liquids due to insects and the like entering the food.
May pots on the fire be left uncovered [in areas that snakes are commonly found or according to those who are stringent]?
Once the pot has been brought to a boil, or has begun to release steam, it can be left uncovered.
May liquids in the fridge or in a closet be left uncovered [in areas that snakes are commonly found or according to those who are stringent]?
What is the law if water that was left uncovered became mixed with other foods/liquids [in areas that snakes are common]?
The liquids are not nullified in 60x.
 Michaber Y.D. 116/1; Michaber O.C. 272/1; Admur O.C. 272/1; Tosefta Terumos 7/13-19; Avoda Zara 30a-b; Chulin 9b
 Tur 116 that a live person who is guarding the liquid will prevent a snake from coming out to drink from it.
 Tur 116; Aruch Hashulchan 116/3
 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 ones floor.
 Michaber Y.D. ibid
 Admur ibid; Michaber O.C. ibid; Some Poskim rule that this includes honey and even ground garlic. [Perisha 116/2]
 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.
Regarding the amount of time that it must be left revealed to be prohibited: Seemingly this includes even a short amount of time, as so long as the snake has enough time to reach it and place venom into it is the amount of time that prohibits it.
 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 Tosafus Avoda Zara 35a] 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]
 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
 Shlah p. 79 “One who guards his soul is distance himself from this” [brought in Elya Raba 170/24; P”M 170 M”Z 10; Pischeiy Teshuvah 116/1; Aruch Hashulchan 116/2; Likkutei Sichos 23/35 footnote 32] Custom of Gra 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.
 The reason: Seemingly this is due to Ruach Raah. [Peias Hasadeh ibid]
 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
 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
 Zivcheiy Tzedek 116/2; Kaf Hachaim 116/9
 Tur and Beis Yosef 116; Hakashrus 18/15
 Kaf Hachaim 272/9; Hakashrus 18/17
 Peri Chadash 116/3; Birkeiy Yosef Shiyurei Bracha 116/5