The following items do not reduce [the opening to less than a handbreadth]: Bone does not reduce if [the source of tumah is] bone; flesh does not [reduce] if [the source of tumah is] flesh; an olive-sized piece of corpse flesh does not [reduce the opening]; an olive-sized piece of carcass does not; a lentil-sized piece of sheretz does not; an eggsized piece of food does not; grain [growing] in the window does not; a very thin spiderweb does not; the carcass of a kosher species of bird that was designated [for food] does not; the carcass of a nonkosher species of bird that was designated [for food] and was ren-dered capable of contracting tumah does not; warp and woof threads that have con-tracted tzaraas do not. Also a brick made from a beis hapras does not; these are the words of R’ Meir. The Sages say: The brick reduces [the opening], because its earth is tahor. This is the rule: Anything that is tahor reduces [the opening], and anything that is tamei does not reduce [the opening].