Rashi on Leviticus 11:33/רש"י על ויקרא י״א:ל״ג

1אל תוכו [AND EVERY EARTHEN VESSEL] WHEREINTO [ANY OF THEM FALLETH SHALL BE UNCLEAN] — An earthen vessel does not become unclean except through the medium of its interior (i. e. only if something unclean is inside it) but not by something unclean touching its exterior) (cf. Chullin 24b).

2כל אשר בתוכו יטמא WHATSOEVER IS INSIDE IT SHALL BE UNCLEAN — the vessel, in turn, renders anything inside it unclean.

3ואתו תשברו AND IT SHALL YE BREAK — This teaches that there is no purification for it by immersion in a ritual bath, (cf. Sifra, Shemini, Section 7 13).