Berakhot 52B

תוכו ואוגנו ואזנו וידיו טהורין. נטמא תוכו ⁠— נטמא כולו.

while its inner side, and its rim, the edge of the vessel that protrudes outwards, and its ear-shaped handle, and its straight handles are pure. However, if the inside of the vessel became ritually impure, it is all ritually impure.

במאי קא מיפלגי?

Although the decrees of Beit Hillel and Beit Shammai are different, they are based on realistic contingencies and on concerns shared by both parties. The Talmud seeks to clarify: With regard to what do they disagree? What is the crux of their dispute?

בית שמאי סברי אסור להשתמש בכלי שנטמאו אחוריו במשקין, גזרה משום ניצוצות. וליכא למגזר שמא יטמאו המשקין שבידים בכוס.

The Talmud explains: Beit Shammai hold: It is prohibited to use a vessel the outer side of which has been rendered ritually impure by liquids. This prohibition stems from a decree of the rabbis, due to concern for drips of liquid that would fall from inside the vessel to its outer side, as those drips themselves would be rendered ritually impure by virtue of their contact with the outer side of the vessel. And Beit Shammai hold that there is no reason to issue a decree due to the concern of Beit Hillel lest the liquid on one’s hands will be rendered ritually impure by the cup, as Beit Shammai hold that the use of a vessel of that kind is prohibited.

ובית הלל סברי מותר להשתמש בכלי שנטמאו אחוריו במשקין, אמרי ניצוצות לא שכיחי, ואיכא למיחש שמא יטמאו משקין שבידים מחמת הכוס.

And Beit Hillel hold: One is permitted to use a vessel the outer side of which has been rendered ritually impure by liquid, as they say: Drips are uncommon, and decrees are not issued on the basis of an uncommon case. Because Beit Hillel permit the use of a vessel of that kind, there is concern lest the liquid on one’s hands will be rendered ritually impure due to the cup.