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מסכת כלים פרק ב משנה ה
כסוי כדי יין An [earthenware] cover for wine jugs
וכדי שמן and for oil jugs
וכסוי חביות נירות and a cover for Neyarian barrels (which is concave)
טהורין are tahor (and cannot become tamei),
ואם התקינו לתשמיש and if he designated them for use (as a receptacle)
טמאים they are [able to become] tamei.
כסוי הלפס The cover of a stew-pot
בזמן שהוא נקוב when it has a hole
ויש לו חדוד or a handle (on its top, preventing it from being able to be set down while upside-down)
טהור is tahor,
אם אינו נקוב and if it does not have a hole
ואין לו חדוד or it does not have a handle
טמא it is [able to become] tamei
מפני שהיא מסננת לתוכו

as she would filter into it

את הירק

the vegetables;

רבי אליעזר בר צדוק אומר Rabbi Eliezer bar Tzadok says
מפני שהיא הופכת עליו because she will overturn onto it
את הרונקי the cooked clump of vegetables.

פרק ב משנה ו

גסטרה A defective vessel (that had split)
שנמצאת בכבשן that was found in a kiln
עד שלא נגמרה מלאכתה [if it broke] before its production was complete
טהורה is tahor,
משנגמרה מלאכתה after its production is complete
טמאה it is [able to become] tamei.
טיטרוס A titros (This is a type of pear-shaped vessel with one hole on the top and smaller pin-sized perforations on its bottom.  One would dip it in liquid, allowing to fill, then block the top hole with one's finger before withdrawing it from the liquid.  The blocked hole on top would prevent the liquid from running out the bottom holes until the finger on top was removed.)
רבי אליעזר בר צדוק מטהר Rabbi Eliezer bar Tzadok rules tahor;
רבי יוסי מטמא Rabbi Yose rules tamei
מפני שהוא כמוציא פרוטות as it drains in droplets (very slowly).


ואם התקינו לתשמיש - As opposed to the case of a pitcher modified for use as a cover (see משנה ג), no physical modification is needed for it to become מקבל טומאה.

מפני שהיא מסננת לתוכו . . . מפני שהיא הופכת עליו את הרונקי - According to the ת"ק, the lid is used to catch liquid and vegetables as they are poured out of the pot, while רבי אליעזר בר צדוק holds that the vegetables from the stew are actually mashed in the lid.  Some say there is no difference between the opinions of the two, and they are just relating what the practice in their region was, however the תפארת ישראל says they argue as follows: רבי אליעזר בר צדוק does not accept the reason of the ת"ק, as the liquid caught in the lid is of no value and is discarded, while the ת"ק rejects the explanation of רבי אליעזר בר צדוק as according to him, the cover need not be concave to serve this purpose.

רבי יוסי מטמא - His opinion is based on two factors: First, it is meant to be used as a container when the top hole is blocked, and so it is considered to have a בית קבול.  Second, it drains so slowly as to not be considered to be "נקוב ככונס משקה", to have a hole within which liquid enters, which would otherwise render it טהור.