Our Gemara on Amud Aleph discusses the concept of Tzamid Pasil, which literally means a tightly sealed cover. The case of Tzamid Pasil is explicit in the Torah, whereby the contents of an earthenware container, (as well as the container itself since earthenware becomes tamey only through its insides), that is wrapped tightly does become tamey when in the dwelling of a corpse. It states in Bamidbar 19:15:

וְכֹל֙ כְּלִ֣י פָת֔וּחַ אֲשֶׁ֛ר אֵין־צָמִ֥יד פָּתִ֖יל עָלָ֑יו טָמֵ֖א הֽוּא׃

⁦and every open vessel, with no lid fastened down, shall be impure.

This verse contains many symbolic words:  impurity, earthenware vessel, which is a metaphor for the body or physicality, and then a tight seal on the lid. All this symbolism lends itself to interesting mystical interpretations to this verse, and I’ll share some highlights:

Be’er Mayim Chayyim (א:א:עח) explains that the word Tefila, prayer, is related to the Tzamid Pasil. (In Hebrew, sometimes letters get switched for example כבש and כשב, as well as שמלה and שלמה). Tefila is the link, bond and seal between the two worlds. 

The Rama in Toras HaOlah (3:65) learns the entire pasuk as a warning: 

⁦And every open vessel, with no lid fastened down, that is every person who has a propensity toward evil and does not try to be attached to Hashem, shall become impure.

Likewise, Agra Dekallah (Chukas 37) explains that the vessel is the mind, as it engages in study and wisdom. If the mind is not attached to God while in study then it is in danger of becoming influenced by impure forces.

And, finally Avodas Yisrael (Parah 6) interprets this verse to be about l’shon Hara. The vessel is the mouth, and if it isn’t sealed then it is likely to to spew l’shon hara.