Before us is Mishnah 7 of Chapter 16 of Maseches Shabbos. This Mishnah deals with the permission to move utensils on Shabbos even for the sake of something that is muktzeh, as long as the utensil being moved is not itself muktzeh.
The text of the Mishnah:
"Kofin ke'arah al gabei haner bishvil shelo techroch bakorah" - one may turn a bowl over upon a flame so that it should not scorch the beam of the house. This is permitted even though the fire itself is muktzeh, provided it is done in a manner that will not extinguish the fire, which is also forbidden.
"Ve'al tzo'ah shel katan" - one may place a utensil over excrement and cover it.
"Ve'al akrav shelo tishoch" - one may place a utensil over a scorpion so that it should not bite.
The view of Rav Yitzchak:
The Gemara cites the opinion of Rav Yitzchak, who holds that "ein kli nital ela l'tzorech davar hanital" - a utensil may be moved only for the sake of something that is not muktzeh. According to his view there is a difficulty: how may one turn over the bowl for the sake of the flame, which is muktzeh? Rav Yitzchak answers that the Mishnah is speaking of a case where the person needs the place where the bowl is lying, and he must move it in any event. Since the bowl is being moved anyway, he may also set it down in a spot where it serves the muktzeh item.
"Ve'al tzo'ah shel katan":
The Gemara clarifies that this is not the Mishnah's intent in its plain sense, for a child's excrement has the status of a chamber pot and a vessel of waste water, which may be moved on Shabbos because of their repulsiveness, and there is no need for the permission of covering them with a utensil. Rather, the Mishnah is speaking of chicken droppings, which are not lying in a place that carries the law of a chamber pot, but rather outside in the courtyard, and there is a concern that the child will play with them. In such a case too it is permitted to cover them, so that the child should not play with them on Shabbos. And the reason the Mishnah says "upon a child's excrement" is that the covering is done for the needs of the child.
"Ve'al akrav shelo tishoch":
This permission stems from the fact that we are dealing with a melachah she'einah tzerichah legufah: the person does not wish to trap the scorpion, but only that it should not bite, and he has no need for the scorpion at all. The Tanna of our Mishnah holds that a melachah she'einah tzerichah legufah - that is, a labor not performed for the purpose for which it was performed in the Mishkan - carries no liability, and there is only a rabbinic prohibition here. And since there is a concern of danger to life, it is permitted to cover it even at the outset.
The opinion of Rav Yehudah:
However, the Mishnah brings the opinion of Rav Yehudah, who holds that one is liable for a melachah she'einah tzerichah legufah, even though it is not performed for the needs of the Mishkan. Rav Yehudah said: there was an incident that came before Rabban Yochanan ben Zakkai in a place called Arav, where they covered a scorpion on Shabbos so that it should not bite, and he said: "choshshani lo mechatas" - I am concerned for him that he may have transgressed a Shabbos prohibition and will need to bring a chatas offering.
In summary: in this Mishnah we learned that one may place a utensil that is not muktzeh over something even for the sake of a muktzeh item: over the flame so that it should not scorch the beam, over the excrement so that the child should not play with it, and over the scorpion so that it should not bite. We saw the view of Rav Yitzchak that "a utensil may be moved only for the sake of something that is not muktzeh," and his answer that the case is one where the person needs the place of the utensil; the Gemara's explanation that "a child's excrement" refers to chicken droppings in the courtyard; and the principle of a melachah she'einah tzerichah legufah - that according to the Tanna of our Mishnah one is not liable for it and it is even permitted at the outset where there is danger, while according to Rav Yehudah one is liable for it, as Rabban Yochanan ben Zakkai was concerned about a chatas.