Terumos, Chapter 8, Mishnah 4. The Mishnah opens an entirely new topic, one that applies to Terumah and to ordinary produce alike: the prohibition of leaving liquids such as water and wine uncovered. The underlying concern is that a snake may have drunk from them and left some of its venom behind, rendering them poisonous to drink.
"Yayin shel terumah shenisgalah - yishafech":
The Mishnah states: "Yayin shel terumah shenisgalah - yishafech" - wine of Terumah that was left uncovered in a way that a snake may have drunk from it and injected its venom, the halachah is that it must be poured out.
The novelty of the Mishnah is that even though it is forbidden to destroy Terumah that is pure, nevertheless it is ruled here that it must be poured out. The reason is the well known principle that danger is treated more stringently than prohibition, and we are stricter with it than we are with prohibitions. Therefore, whenever there is any concern of danger and poisoning, not only is it permitted to pour out the wine and be rid of it, it is an obligation to do so and not to drink it.
The Gemara notes that someone who owns a cat may feed this liquid to the cat, since cats are immune to snake venom.
The Mishnah continues: "V'ein tzarich lomar shel chulin" - it goes without saying that if this were not Terumah wine but ordinary, non-sacred wine, it may not be drunk and must be poured out.
Three liquids forbidden because of being left uncovered:
"Sheloshah mashkin asurin mishum gilui" - three types of liquid become forbidden if they were left uncovered in a way that a snake could have reached them, and they must be poured out:
"Hamayim" - plain water.
"V'hayayin" - wine, and likewise grape juice.
"V'hechalav" - milk.
The Gemara adds further liquids to this list: honey, crushed garlic mixed with liquid, fish brine, and similar items.
By contrast, "ush'ar kol mashkin mutarin" - all other liquids, such as juices and light beverages, are permitted even if they were left uncovered, since snakes do not drink from them, and there is no reason to suspect that a snake drank from them.
How long an exposure creates the prohibition?
"Kamah yish'hu v'yihyu asurin? K'dei sheyeitzei harechesh mimakom karov v'yishteh" - how long must liquids stand uncovered before they become forbidden on account of gilui (being left uncovered)? A very short span of time suffices, enough for a snake hiding nearby to come out and drink. "Rechesh" means the crawling creature, that is, the snake, and "mimakom karov" means from even the closest hiding place, such as beneath the handle of the pot or the cup. It is enough that it come out, take one quick sip and disappear, for the wine, water and the like to become forbidden on account of gilui.
In summary: this measure of time is not long at all, and therefore, according to the plain sense of the halachah, any wine, water or milk left uncovered in a way that a snake could have drunk from it is forbidden immediately, even wine of Terumah, which is likewise poured out, because danger is treated more stringently than a prohibition.