Kelim, Chapter Three, Mishnah Five
1) One who lines a sound vessel: Rabbi Meir and Rabbi Shimon say: [the lining] contracts impurity.
2) But the sages say: a lining over a sound vessel is not susceptible to impurity, and only one over a cracked vessel is susceptible.
3) And the same dispute applies to the hoop of a pumpkin shell.
Section one: The debate in this mishnah concerns a person who lined the outside of a vessel that was in sound shape. According to the Tosefta, he did this so that he could cook with this vessel.
Rabbi Meir and Rabbi Shimon rule that if the vessel becomes defiled from the inside (earthenware vessels can only be defiled from the inside), the lining on the outside is also impure. The lining can then subsequently defile any food or drink that touches it on the outside.
The other sages say that the lining is susceptible to impurity only if it is necessary to hold the vessel together. So the lining of a sound vessel is not susceptible, whereas the lining of a cracked vessel is.
Section two: Dried pumpkin shells were used to draw water. They would attach a hoop to the shell in order to strengthen it. This is similar to the previous case, in that both are cases of strengthening a vessel that is in sound shape. According to Rabbi Meir and Rabbi Shimon the hoop is susceptible to impurity, whereas the sages hold that it is not.