Shabbat 58B

ושל דלת ⁠— טהור.

and the bell of a door is ritually pure. The door itself is not considered a vessel. It is considered part of the house, and therefore its status is like that of the house. The house is attached to the ground, and therefore it cannot become ritually impure. Everything connected to it, including the bell, assumes that status.

של דלת ועשאו לבהמה ⁠— טמא. של בהמה ועשאו לדלת, אף על פי שחברו לדלת וקבעו במסמרים ⁠— טמא, שכל הכלים יורדין לידי טומאתן במחשבה, ואין עולין מידי טומאתן אלא בשנוי מעשה!

If one took the bell of a door and converted it into a bell for an animal, it can become ritually impure; however, if one took the bell of an animal and converted it into a bell for a door, even though he attached it to the door and even fastened it with nails, it can still become ritually impure because all utensils descend into their state of ritual impurity by means of thought alone, i.e., as a result of a decision to designate them for a specific purpose through which they will become susceptible to ritual impurity, they receive that status immediately. However, they only ascend from their state of ritual impurity by means of an action that effects physical change to the vessel itself. A change in designation alone is ineffective. This baraita states that an animal bell can become ritually impure, contrary to that which was taught in the previous baraita.

לא קשיא: הא דאית ליה עינבל, הא דלית ליה עינבל.

The Talmud answers: This is not difficult. This baraita, where it was taught that the bell can become ritually impure, is referring to a case where it has a clapper [inbal]. That baraita, where it was taught that the bell cannot become ritually impure, is referring to a case where it does not have a clapper.