Numbers 19:15

וכל כלי פתוח AND EVERY OPEN VESSEL — Scripture is speaking of an earthen vessel which is not receptive of uncleanness by an unclean thing touching its exterior but only by it being in its interior (even without contact with the interior walls; cf. Rashi on Leviticus 11:33); therefore if the cover that forms its lid is not well joined to it by a perfect contact, it (the vessel) becomes unclean; but if the lid is joined upon it, it is clean, whilst metal vessels would in any case become unclean, whether they are closed or not, since they are receptive of uncleanness whether an unclean object touches them on their inside or on their outside (Chullin 25a). פתיל is an expression denoting “joined” in the Arabic language. Similar is (Genesis 30:8; cf. Rashi thereon), נפתולי אלהים נפתלתי, “By bonds of God I have joined myself (נפתלתי) with my sister."

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

And every open vessel, which has no covering bound upon it, is unclean.