Shabbos, Chapter 17, Mishnah 6. This mishnah deals with two vessels used for drawing water, and with the question of when they may be used on Shabbos even though something muktzeh has been attached to them.
"Ha'even shebakruyah":
A kruyah is a dried gourd, which was sometimes used as a small vessel for drawing water from a cistern or a well. Since a dried gourd is light in weight, it does not sink into the water on its own, and therefore a stone would be placed inside it to serve as a weight.
In such a case the mishnah distinguishes between two situations:
"Im mema'lin bah v'einah nofeles - mema'lin bah" - when the stone is tied and fastened firmly, so that it does not fall out while water is being drawn, one may fill it with water on Shabbos.
"V'im lav - ein mema'lin bah" - when the stone is loose and liable to fall out, one may not draw water with it on Shabbos.
The reason for the distinction: the law of muktzeh and of a base:
The stone is muktzeh, and because of it the entire gourd may not be handled. Even if the stone were to fall out on Shabbos it would still be forbidden, because the gourd has become a base for the stone, that is, a holder for it, and therefore it takes on the stone's status: the stone is muktzeh, and the gourd is muktzeh as well, since the stone was deliberately placed in it to serve as a weight. But when the stone is tied firmly so that it cannot fall out, it is nullified and becomes secondary to the gourd, and the gourd may be handled on Shabbos.
"U'zemorah shehi keshurah b'tafiach":
A zemorah is a vine branch, which serves as a kind of rope, and it is tied to a tafiach - a small earthenware vessel used for drawing water from a spring or a well. In this case one may draw water with it on Shabbos, and it is permitted to lower the vessel into the water by means of the branch.
The reason: the branch is nullified and considered secondary to the vessel that is being used, and therefore it has no status of muktzeh. Although in itself it resembles the stone, since ordinarily it has no function or use, here it has been given a function and it is considered part of the tafiach, part of the earthenware vessel, and therefore it is not muktzeh on Shabbos.
In summary: In this mishnah we learned the relationship between a muktzeh item and the vessel used together with it. A stone in a gourd: if it is loose and liable to fall out, the gourd becomes a base for the stone and may not be handled; but if it is tied firmly, it is nullified to the vessel and one may draw water with it. So too a vine branch tied to an earthenware jug: although in itself it has no use, once it has become part of the vessel it is nullified to it and is not muktzeh.