Kelim, Chapter Two, Mishnah Five
Our mishnah deals with the covers of various types of vessels.
1) The covers of wine jars and oil jars and the covers of papyrus jars are not susceptible to impurity
a) But if he adapted them for use as receptacles they are susceptible.
2) The cover of a pot:
a) When it has a hole or it has a point, it is not susceptible to impurity,
b) But if it does not have a hole or a pointed top it is susceptible because she drains the vegetables into it.
i) Rabbi Eliezer bar Zadok says: because she turns out the contents [of the pot] on to it.
Section one: Generally, the covers of wine, oil and papyrus jars are not susceptible to impurity because they are not usually used as receptacles. However, if one fixes them in such a way that they can be used as receptacles, then they take on the characteristic of being able to receive impurity.
Section two: If the cover of a cooking pot has a hole in it, or has a pointed lid, then it cannot be stood on its reverse side so as to contain the contents of the pot. Since it can’t be used as a receptacle, it is not susceptible to impurity.
However, if it does not have a hole or a pointed top, the person (assumed by the Mishnah to be a woman) might use the top as a colander for her vegetables (the lid does not have sides, so the water goes out the side). Since it holds the vegetables, it is susceptible to impurity.
Rabbi Eliezer bar Zadok agrees that in this case the lid is susceptible to impurity but provides a slightly different explanation. When the woman turns over the pot, she uses the lid to hold in the contents of the pot while the water goes out the side. I think that this is something we still do when we make pasta.