A clay vessel can become clean only if it is touched on the inside, not on the outside. Also, if it is not touched, but the contaminating article is merely inside its space, it is still contaminated (Rashi; Chullin 24b; Sifra; Yad, Kelim 13:1).
|anything inside it...|
Primarily food and drink, as next comment, but not other vessels (Sifra; Pesachim 20b; Yad, Kelim 13:3).
|shall be broken|
A clay vessel thus becomes clean when broken (Yad, Kelim 19:1). It cannot, however, be purified by immersion (Sifra; Yad, Mikvaoth 1:3). See Leviticus 6:21.