Tevul Yom, Chapter 2, Mishnah 8. A tevul yom is someone who contracted tumah, already immersed in a mikveh, and is now waiting only for nightfall. In that interim state he invalidates terumah that he touches, though chullin liquids are unaffected by him. This chapter examines what his touch reaches when it lands on one point of a larger body of food or liquid. Our mishnah takes up two such cases: a barrel of terumah wine that has sprung a leak, and a stream of liquid in mid-pour.
The opening case: "Chavis shenikvah", a barrel of terumah wine has been pierced. Three possible locations for the puncture are listed: "bein mipiha", at its mouth; "bein mishuleha", down at its base; "bein mitzideha", in its wall. "Venaga bah tevul yom": a tevul yom laid his hand on the wine exposed at that opening. The verdict is "temei'ah": every drop the barrel holds has become tamei. No matter where the breach happens to sit, the wine visible there counts as part of the entire contents, so contact with it amounts to contact with all of it.
Rebbi Yehudah rules otherwise, drawing a line according to where the breach sits. "Mipiha umishuleha, temei'ah": a puncture at the top or down at the base transmits the contact, and everything the barrel holds is rendered tamei. "Min hatzdadin mikan umikan, tehorah": a breach in the wall, whether to this side or to that, is another matter; contact there leaves the contents tahor, because such a touch is not viewed as extending to the wine standing within.
The mishnah's second half turns to decanting. "Hama'areh mikli lechli": liquid is being poured out of one vessel into another below it. "Venaga tevul yom bekiluach": while the stream was still running between the two vessels, a tevul yom made contact with it. "Im yesh bo": should the tahor liquid already gathered in the lower vessel come to a sufficient measure, "ya'aleh be'echad ume'ah", the small quantity his hand reached is nullified at a proportion of one part in a hundred and one, and it is botul. Should the receiving vessel not hold that measure of tahor liquid, there is nothing to nullify the contact, and everything collected in it is disqualified.
Two lessons emerge together here. A touch at an opening can be treated as a touch upon everything the vessel holds, and a touch upon a stream in mid-air reaches the liquid it is joining below, unless there is enough tahor liquid there for the small amount he touched to be swallowed up and nullified within it.