Eruvin 20B

וכן בגת.

And the law is likewise in a winepress with respect to tithes. As long as one’s head and most of his body is in the winepress, he may drink from the wine without first separating tithes because drinking wine in a winepress is considered incidental drinking, which does not require tithing. The activity of harvesting and ingathering grapes is not considered completed as long as the grapes are in the winepress, since the grapes are still intended for making wine. Consequently, at this stage one may consume the produce in a casual, incidental manner. Once the work has been completed, however, one must tithe the produce before consuming any of it.

גבי אדם ⁠— הא אמר דבעי ראשו ורובו. גבי פרה ⁠— מי בעינן לה ראשה ורובה או לא?

The Talmud clarifies several laws related to this issue: With regard to a person, it was said in connection to these laws that it is necessary that his head and most of his body be inside the domain from which he is drinking. However, a question may be raised with regard to a cow standing in a public domain and drinking from a private domain, or vice versa: Is it necessary that its head and most of its body be inside the domain from which it is drinking, or not?

כל היכא דקא נקיט מנא ולא נקיט לה ⁠— לא תיבעי לך דבעי ראשה ורובה מלגיו. כי תבעי לך ⁠— היכא דנקיט מנא ונקיט לה, מאי?

The Talmud clarifies: Wherever one holds the bucket from which the cow is drinking but does not hold the animal, there should not be a dilemma for you, for it is certainly necessary for its head and most of its body to be inside, as the cow might move backward and pull the bucket with it, causing him to carry it from one domain to the other. Where there should be a dilemma for you is where he holds the bucket and also holds the animal. What is the law in such a case?