Parah, Chapter 12, Mishnah 9. The closing mishnayos of this chapter examine the law of chibur, connection: when are two parts of an object treated as one unit? The question has two separate applications, and they do not always give the same answer. There is chibur with regard to tumah, meaning that tumah reaching one part spreads to the other, and there is chibur with regard to hazayah, the sprinkling of the mei chatas, meaning that sprinkling on one part purifies the whole. Our Mishnah lists a series of items that are a connection for the first purpose but not for the second.
The list
"Hatzalim shebakantel": a kantel is the wooden carrying frame strapped to a donkey's back, and the tzalim are the baskets that hang from it, one on each flank.
"Vehamitah shel tarbal": the threshing couch, the frame on which the worker sits or reclines while threshing.
"Vekeren shel keliva": the corner piece of a keliva, the bier used for carrying a meis at a funeral.
"Ukranin shel yotzei derachim": the horns carried by travelers, used for drawing water and drinking from it.
"Veshasheles hamaftechos": the chain on which keys are strung.
"Ushlal hakovsim": the loose basting stitches put in by launderers.
"Vehabeged shehu tafur bekilayim": a garment whose pieces are sewn to one another with kilayim, wool and linen together.
The ruling
Concerning all of these the Mishnah states: "chibur letumah ve'eino chibur lehazayah." They are a connection for tumah, so that if one part contracts tumah the other parts become tamei along with it, precisely because they are viewed as joined. But they are not a connection for sprinkling. It is not enough to direct the mei chatas at one of the parts and assume that the whole has been purified; each part must receive the sprinkling in order to become tahor.
The Mishnah is thus teaching a measured distinction. These are joinings loose enough that they do not turn the pieces into one true utensil for the purposes of taharah, yet real enough that tumah passes freely across them. Where tumah is concerned the Torah is stringent and treats the pieces as one; where the removal of tumah is concerned the sprinkling must reach every part on its own.