Gittin 16A

הנצוק, והקטפרס, ומשקה טופח ⁠– אינו חיבור לא לטומאה ולא לטהרה!

A stream of water, and water descending an incline [katafres], and liquid that rendered an item moist do not connect, neither for ritual impurity nor for purity. These liquids do not connect for impurity, e.g., if impure water is in one place and becomes attached to another source of water pouring from above it, the water above is not considered attached to the impure water and is not rendered impure. They also do not connect for purity, e.g., if two collections of water are attached via the pouring of a stream, they do not join together to form the amount of water necessary to form a valid mikveh, through which people and items can become ritually pure. If so, liquid that rendered a hand moist should not serve to attach the two halves of one’s hand when they are washed.

לא צריכא, דאיכא טופח להטפיח. הא נמי תנינא: טופח להטפיח ⁠– חיבור!

The Talmud answers: No, this dilemma is necessary in a case where there is liquid that rendered an item moist enough to render another item moist. In other words, the half of the hand that was washed is not merely slightly moist, it is moist enough to render something else moist. The Talmud asks: We already learned this as well: With regard to liquid that rendered an item moist enough to render another item moist, it connects with other liquid. What, then, is Ilfa’s dilemma?

דלמא לענין מקואות, ורבי יהודה היא. דתנן: מקוה שיש בו ארבעים סאה מכוונות, וירדו שנים וטבלו (שניהם בבת אחת ⁠– טהורים),

The Talmud suggests: Perhaps the halakha that liquid rendered an item moist enough to render another item moist joins with other liquid is stated only with regard to the issue of ritual baths, and this is in accordance with the opinion of R' Yehuda. As we learned in a Mishnah (Mikvaot 7:6): In the case of a mikveh that contains precisely 40 se’a of water, and two people descended and immersed in it, if both of them immersed at the same time they are ritually pure.