Mishnah 3 in chapter 17 of Maseches Shabbos deals with the "olive reed." This is a hollow reed, a marsh reed, that was used in the processing of olives: when the olives reached the stage of ripening, they would be placed in a vessel called a ma'atan, where they ripened over a period of time. With that reed they would turn the olives over and check whether they had ripened sufficiently, to the point where oil could be pressed from them.
Two questions come up for discussion regarding this reed: whether it is susceptible to tumah, and whether it may be moved on Shabbos.
The law of tumah:
"Im yesh kesher b'rosho - mekabel tumah" - at times they would insert into the reed a sort of plug or knot. Once a cavity is formed in the reed that is capable of holding something, it has a receptacle, and a bit of oil is liable to remain in it, held back by that knot. It is therefore considered a vessel and is susceptible to tumah.
"V'im lav - eino mekabel tumah" - when the reed has no knot, it has no receptacle, and it is not susceptible to tumah.
The law of moving it on Shabbos:
Even so, the Mishnah rules: "Bein kach u'vein kach nital b'Shabbos" - whether it has a knot or not, whether it has a receptacle or not, it may be moved on Shabbos, because it has the status of a vessel. And although it is a vessel whose function is for a forbidden use, since it serves the olive pressing process which is forbidden on Shabbos, nevertheless it may be moved for its own use or for the sake of its place: when a person needs the object itself, or when he needs the place where the object is lying.
In summary: the olive reed is susceptible to tumah only when there is a knot at its top, which gives it a receptacle; but as for moving it on Shabbos - in either case its status is that of a vessel whose function is for a forbidden use, which may be moved for its own use and for the sake of its place.