Uncovered Drinks

Hello Would tiny or very small holes on the cover af a vessel create of problem of מים מגולים for the water found in that vessel?

Shalom!

Thank you for your question.

In ancient times, when snakes and scorpions were common in residential areas, the sages enacted a ban on consuming liquids that had been left uncovered and unsupervised lest a poisonous snake or scorpion may have emitted venom into the drink. If one were to drink it one could die.

The ban on drinking uncovered and unsupervised liquids is known as “mashkim megulim” which means “uncovered beverages.” Although the ban is found in the Talmud and advocated by the Rambam, the Shulchan Aruch rules that the ban is no longer binding nowadays since such poisonous creatures are not common in residential areas anymore. The halacha is in accordance with this view.

Nevertheless, there are some authorities who advocate maintaining the enactment even nowadays. This is because it is argued that although uncommon, poisonous snakes do exist, and the requirement to take heroic measure to preserve life, no matter how minimal the risk, call for us to observe the enactment even nowadays. There is also much discussion as to if, and when, a rabbinic enactment may be cancelled in situations where the stated reason for the enactment is no longer relevant. That is a discussion beyond the scope of this answer.

As mentioned, the halacha is that one is not required to observe the ban, and one is permitted to drink beverages that were left unsupervised even for many hours (the only exception is wine for kiddush). One who wishes to observe the ban should cover the beverages in a way that does not allow snakes, and the like to enter them. 

Therefore, as to your question regarding a cover with tiny holes: if it is not possible for a snake to enter due to the size of the holes which is the size of a small finger of a newborn baby, then the cover suffices to comply with the enactment of mashkim megulim.

Avodah Zarah 30a, Chullin 9b; Terumos 8:4; Y.D. 116:1 and commentaries