The sages thought that snakes drop venom into whatever they eat or drink. Therefore they forbade eating a fig whose end is bitten off, lest a snake have made that bite and left behind venom, which would be a danger to the person who ate the fig. Thus, too, for water, milk, and wine — if they were in an uncovered vessel, the sages feared a snake may have drunk from them and left venom behind. Therefore the sages forbade drinking these liquids if they were open for a snake to drink from. But snakes do not eat of every food or drink of every liquid. Food and drink which snakes do not like is permitted for eating. A discussion then arose about what drinks the snake customarily drinks. We have been told, for example, that the snake does not drink diluted wine.
As to wine diluted with water, one sage argued that the snake does not drink wine diluted with a great deal of water, but if it is diluted with only a little bit of water the snake will drink from it. Scholars questioned this, for there was once an incident in which two sages were traveling by boat, and there were pitchers of wine in the boat as well. Suddenly they saw a snake and to drive him off the sages put a little bit of water in a pitcher of wine; the snake did indeed veer off. From this the scholars learned that the snake does not customarily drink wine diluted with a little bit of water. Other scholars rejected this conclusion, claiming that the snake left because he was afraid; snakes do not customarily endanger their lives for diluted wine (though the snake might endanger its life to drink undiluted wine), but if there had been no one on board the ship, the snake definitely would have drunk the diluted wine.
The scholars brought another proof that the snake does customarily drink wine diluted with water: some sages sat and drank wine diluted with water. After they finished drinking some undiluted wine remained in the pitcher. They covered the pitcher with a rag. Suddenly the sages saw a snake with a mouth full of water. The snake ejected the water into the wine, through the cloth, until the wine overflowed the cloth and then the snake began to drink it. From this story the scholars concluded that the snake does drink wine diluted with water. Others came and rejected this claim, stating that there is a difference between wine which has been diluted by man and wine which, as in this case, the snake diluted on his own.
The Talmud does reach a decision on this issue. It rules that the snake customarily drinks wine diluted with water, and that therefore one is forbidden to drink diluted wine if it sat uncovered and unguarded.
(Babylonian Talmud, Tractate Avodah Zarah 30a)
The Talmudic text:
The question was asked: Is boiled wine rendered forbidden by being left uncovered or is it not so? Come and hear: Rabbi Jacob the son of Idi testified in regard to boiled wine that it is not rendered forbidden by being left uncovered.
Rabbi Yannai the son of Ishmael was sick and R’ Ishmael the son of Zirud and other rabbis called to enquire about him. As they sat, the question was asked of them: Does the objection to remaining uncovered apply to boiled wine or not? To which R’ Ishmael the son of Zirud replied: Thus said Rabbi Simeon the son of Lakish on behalf of a great man, namely R’ Chiyya: Boiled wine is not rendered unfit by being left uncovered. On their asking, “Shall we rely on it?” R’ Yannai the son of Ishmael motioned [as if to say], “Upon my responsibility.” Samuel and Ablet were sitting together when boiled wine was brought up for them and [the latter] withdrew his hand, but Samuel said to him: Behold, it has been said that boiled wine is not to be suspected of idolatrous use! Rabbi Chiyya’s maid-servant found that some boiled wine had been left uncovered. She came [to ask about it] of R’ Hiyya, who told her that it had been declared that boiled wine is not rendered unfit by being left uncovered. The servant of Rav Adda the son of Ahavah found that some diluted wine had been left uncovered. [His master], however, told him that it had been stated that diluted wine is not rendered unfit by being left uncovered. Rav Papa said: This has only been said [of wine] that is well diluted; but if it is only slightly diluted [a snake] might indeed drink it. But does it indeed drink wine that is slightly diluted? [What about] Ravah son of Rav Huna who was traveling in a boat and had some wine with him? Observing that a snake, cutting through the water, was approaching, he said to his attendant, “Turn it away,” and the attendant took some water and was pouring it into the wine; whereupon the snake turned back! [This may only show that] for pure wine [the snake] will even endanger its life, while for diluted wine it will not face danger. And does it not face danger for diluted wine? What about Rabbi Yannai who was at Akbara (some say it was Bar-Hadaya that was at Akbara) where people were sitting and drinking diluted wine, and as there was some of it left in the cask they tied a shred of cloth over it? He then saw a snake carrying water which it poured into the cask till the cask was so filled that the wine came above the shred of cloth, and [the snake then] drank! It may be said that what [the snake] itself dilutes it will drink, but it will not drink what others dilute. Said Rav Ashi (some say, Rav Mesharsheya): What an answer [to give in a matter] where danger [is involved]!