וְכֹל כְּלִי פָתוּחַ אֲשֶׁר אֵין צָמִיד פָּתִיל עָלָיו טָמֵא הוּא
Any open vessel which has no seal secured on it is impure
The Torah discusses the laws of how tum’ah, impurity, can spread from a corpse. One of the ways it spreads is that when the corpse is indoors, anything in the same room becomes impure as well. There is one exception to this. Earthenware vessels, unlike any other, become and spread impurity only from their insides. (This is unlike most other types of vessels where even touching the outside of it can render it or the one who touched it impure.) Therefore, the Torah tells us that if the vessel is sealed, it remains pure, (because although the airspace of the room is impure, the inside of this vessel is protected by the seal) but if it is open, the impurity can enter, and the vessel is impure.
The Chafetz Chaim writes that we can take a lesson from this for ourselves. We are created from earth, and thus are likened to an earthenware vessel. If we keep our mouth sealed when we should, we are like a sealed vessel, and no impurity can invade us. If, however, we open our mouth when we shouldn’t, i.e. speaking lashon hara or other things we shouldn’t say, we “broke open the seal” and now the impurity has free rein over us.
We should all merit to know how and when we should keep our mouth closed and stay a holy nation.