Kelim, Chapter Fourteen, Mishnah Four
The next two mishnayot deal with the parts of an ox-pulled wagon that are susceptible to impurity and the parts that are not.
The parts of a wagon that are susceptible to impurity: the metal yoke, the cross-bar, the side-pieces that hold the straps, the iron bar under the necks of the cattle, the pole-pin, the metal girth, the trays, the clapper, the hook, and any nail that holds any of its parts together.
Rather than explain what each of these things exactly is (I’m not even sure I could explain), I think it is better to simply point out that all of these vessels serve indispensable functions on the wagon. Without them, the wagon will not function properly. Therefore, they are all susceptible to impurity. In other words, functional and necessary vessels are susceptible to impurity.