Kelim, Chapter Ten, Mishnah Three
1) A stopper of a jar that is loose but does not fall out:
a) Rabbi Judah says: it protects.
b) But the sages say: it does not protect.
2) If its finger-hold was sunk within the jar and a sheretz was in it, the jar becomes unclean.
a) If the sheretz was in the jar, any food in it, become unclean.
Section one: According to Rabbi Judah, although the stopper is loose in its place, it still protects the jar from becoming impure. The sages disagree.
Section two: The finger-hold is the depression within the stopper used to get a grip on the stopper and pull it out. If there was a sheretz in this finger-hold and it was sunk within the jar, then the jar is unclean. The finger-hold does not count as sealing the jar, even though the stopper does. Similarly, if the sheretz was in the jar and the finger-hold was sunk into the jar and there was food in the finger-hold, the food is impure. Put simply, the finger-hold does not protect from impurity.