Shabbos, Chapter 20, Mishnah 1. This mishnah deals with machshirei ochel nefesh - items needed for preparing food on Yom Tov, which one is permitted to do on Yom Tov. The question under discussion concerns machshirei ochel nefesh that could have been done even before Yom Tov: what is their status on Yom Tov itself?
As we saw in the previous chapter regarding circumcision, Rabbi Eliezer held that one may perform all the necessary preparations, even those that could have been done before Shabbos. He rules the same way in matters of food preparation: even though the preparation could have been done before Yom Tov, it is permitted to do it on Yom Tov.
What is the mishmeres:
The mishnah discusses hanging a mishmeres - a kind of basket or strainer that people used to remove the sediment from wine, which they would hang over the vessel. Hanging it raises a concern of making a tent or of fashioning a vessel, both of which are considered forbidden on Shabbos and on Yom Tov. Since it could have been set up over the vessel even before Yom Tov, the question arises: if he did not do so, may he hang it on Yom Tov?
The view of Rabbi Eliezer:
"Tolin es hamishmeres b'Yom Tov" - one may hang the strainer on Yom Tov, even though it could have been done before Yom Tov.
"V'nosnin l'teluyah b'Shabbos" - if a strainer was already hanging over a vessel from before Shabbos, one may pour wine into it and strain it on Shabbos. Although this involves a kind of selecting and separating, this is not the normal manner of borer, and therefore it is permitted.
The view of the Sages:
"V'Chachamim omrim: Ein tolin es hamishmeres b'Yom Tov" - one may not hang the strainer over the vessel on Yom Tov, since it could have been done before Yom Tov.
"V'ein nosnin l'teluyah b'Shabbos" - one may also not strain wine on Shabbos in a strainer that is already hanging over the vessel, for in the view of the Sages this is a form of borer, a separating that is forbidden on Shabbos.
"Aval nosnin l'teluyah b'Yom Tov" - on Yom Tov one may pour wine into the hanging strainer, since this falls under the category of ochel nefesh, food preparation that is permitted on Yom Tov.
In summary: in this mishnah Rabbi Eliezer and the Sages disagree about the law of machshirei ochel nefesh that could have been done before Yom Tov. Rabbi Eliezer permits hanging the mishmeres on Yom Tov, and even permits pouring wine into a hanging strainer on Shabbos, because this is not the normal manner of borer. The Sages forbid the hanging on Yom Tov because it could have been done before Yom Tov, and they forbid pouring into a hanging strainer on Shabbos because of borer, but they permit pouring into a hanging strainer on Yom Tov on the grounds of ochel nefesh.