Shabbos, Mishnah 6. This mishnah presents some of the first laws of muktzeh - items that may not be moved on Shabbos. But it opens with a rather distinctive halachah in this area: one may not place a vessel beneath a lamp that is dripping oil, in order to catch the dripping oil and save it.
Why placing the vessel was prohibited:
The oil dripping from the lamp is muktzeh: since it was set aside for the purpose of Shabbos lighting, there is no use for it. Even while it was still in the lamp it could not be used, since removing the fuel amounts to extinguishing. It was set aside for that purpose, and therefore it may not be used.
And when the oil falls into the vessel, the vessel is prevented from its original use, and it too becomes forbidden to move on Shabbos. This is a rabbinic decree, for two reasons:
It looks like destroying the vessel on Shabbos, something akin to breaking a vessel.
It looks like fixing the vessel and setting it in place, a sort of form of building - as though it were glued down where it stands, for beforehand it could be moved, and from now on it may no longer be moved because of the muktzeh inside it.
However, if the vessel was put in place before Shabbos, it is permitted, since here there is no act of disabling a vessel's use performed on Shabbos itself.
Even so, one may not benefit from the oil on Shabbos and may not use it, because it is not "muchan" - prepared - which is another way of saying that it is muktzeh. Muktzeh is the opposite of muchan: something prepared while it was still day, before Shabbos, is not muktzeh, whereas something that is not prepared has been dismissed from one's mind and from any use, and that is what is called muktzeh.
Moving a new lamp and a used lamp:
The mishnah continues with a further law of muktzeh: one may move a new lamp, a new lamp that has not yet been used for lighting. Even though it was designated to serve as a lamp, it may be used as a vessel for other purposes and other items may be placed in it.
But a lamp that has already been used may not be moved, even if it is not being used for lighting this Shabbos. Since it was lit in the past, it falls under the law called muktzeh because of repulsiveness. That is how lamps were designed in their day: after being used they became sooty and repulsive, and one does not want to touch them. True, in case of need one could touch it and wash one's hands afterwards, but ordinarily one has no desire to do so, and therefore this is muktzeh because of repulsiveness and it may not be moved on Shabbos.
The view of Rabbi Shimon:
Rabbi Shimon disagrees and holds that all lamps may be moved, even those that have already been used, and not only a new lamp. He is not concerned about muktzeh because of repulsiveness in general, for Rabbi Shimon is the most lenient opinion in the laws of muktzeh in all of the Talmud.
Even so, there is an exception: a lamp that is actually burning on Shabbos. A lit lamp may not be moved, and there are two reasons for this:
It might be extinguished by the movement.
It becomes a basis - from the word foundation - for a forbidden item, namely the flame. Something that is not muktzeh in its own right, but serves as a support for a muktzeh item, may not be moved; and the lamp supports the flame, which is muktzeh.
And because of the very concept of muktzeh - something set aside from a person's mind and from its intended use - even after the flame has gone out the lamp may not be moved for the remainder of that Shabbos.
A vessel to catch sparks:
The mishnah returns to the subject with which it opened: although one may not place a vessel beneath the lamp to collect the dripping oil, because the vessel becomes fixed in place and unable to be moved, one may place a vessel beneath the lamp to catch falling sparks, in order to protect something from catching fire - and this may be done even on Shabbos itself.
The reason: the sparks in themselves create no problem and do not prevent the vessel from being moved, since they do not remain in it at all but are extinguished immediately. There is no muktzeh item sitting in the vessel after the sparks have fallen - they go out and disappear.
However, water may not be put into that vessel, not even on Friday afternoon, because of a concern about extinguishing: we are concerned that on Shabbos the vessel with the water in it might be lifted just as the sparks are falling, and their extinguishing would be hastened slightly compared with when they would have gone out naturally. He would then be actively extinguishing the sparks, instead of letting them go out on their own. Therefore water may not be put into it even on Friday afternoon.
In summary: in this mishnah we learned the fundamentals of muktzeh: the oil dripping from the lamp is muktzeh, and therefore one may not place a vessel beneath it on Shabbos - both because it appears to be breaking a vessel and because it appears to be fixing it and setting it in place - whereas before Shabbos it is permitted, though one may not benefit from the oil because it is not "muchan." We also learned the distinction between a new lamp, which may be moved, and a used lamp, which may not because of muktzeh due to repulsiveness; the lenient view of Rabbi Shimon, except regarding a lit lamp, which is a basis for a forbidden item; and the permission to place a vessel to catch sparks - provided that no water is put in it, not even from Friday afternoon, out of concern for extinguishing.