Mishnah 2 in chapter 20 of Maseches Shabbos deals with cases that appear to involve the melachah of borer - separating waste from food - but in fact contain no separation forbidden on Shabbos. The Mishnah lists four such cases, and at the end it adds a matter that has nothing to do with borer at all, but rather with the laws of exertion on Shabbos.
"Nosnin mayim al gabei shemarim bishvil sheyitzolu":
One may pour water onto the grape sediment at the bottom of a wine barrel, so that the wine still left within the sediment will be absorbed into the water and give it the taste of wine. Although it appears to the eye as though he is separating the wine from the sediment, this is not separation at all, since his entire action amounts to soaking the sediment in water, and there is no act of separating here.
"U'mesanenin es hayayin b'sudarin uv'chefifah mitzris":
One may strain wine through a cloth, and likewise through a kefifah mitzris - an Egyptian basket that also serves as a strainer. There are three reasons for the permission:
There is no concern of squeezing: The Mishnah is speaking of a cloth designated specifically for this purpose. With a garment that is not meant to get wet, there would be room to worry that he might come to wring it out, but with a cloth designated for this, we are not concerned.
There is no borer here: Even though he is straining, the wine is fit to drink even without straining, and the straining is merely an improvement in the quality of the taste. For this reason it is not considered borer.
A shinui: It must be done in a manner somewhat different from the weekday way. The Gemara says that when setting up the cloth one should not form a hollow in it - a kind of depression - but rather lay it out flat. In this way it is evident that the act is being done with a change, and it is clear that there is no violation of the prohibition of borer here.
"V'nosnin beitzah b'masnenes shel chardal":
One may place an egg in a strainer used for straining mustard and its ground seeds. Although a strainer normally collects the waste that a person does not want, here the permission stems from the purpose: as the Gemara says, the intent is to give the mustard color from the yolk of the egg, and this color is imparted equally both above the strainer and below it. It turns out that there is no intent to strain the egg at all, only to add color to the mustard seeds.
"V'osin anumlin b'Shabbos":
This matter has no connection to borer, but rather to the question of exertion - something that involves a person troubling himself. Anumlin is a mixture of wine, honey and pepper, a beverage that people were accustomed to prepare. The Mishnah permits preparing it on Shabbos, even though it involves a certain amount of exertion, and even an unusual amount of exertion.
The Tannaim disagree about the permitted measure:
Rabbi Yehudah: There is a limit on the quantity. On Shabbos - by the cup, meaning a single full cup and not a large quantity; on Yom Tov - by the lagin; and on Chol HaMoed - by the barrel.
Rabbi Tzadok: "Hakol lefi ha'orchin" - the measure is determined by the number of guests, that is, according to the need before him. Making more than what is needed involves exertion that is unnecessary on Shabbos.
In summary: In this Mishnah we learned four cases that look like borer but are not borer: pouring water onto the sediment, which is nothing more than soaking; straining wine through a cloth or an Egyptian basket, which is permitted because there is no concern of squeezing with a cloth designated for this, and because the wine is fit to drink even without straining, provided it is done with a change; and placing an egg in a mustard strainer, whose purpose is color and not straining. Finally we examined the making of anumlin, which is not a matter of borer but of exertion, and the dispute between Rabbi Yehudah and Rabbi Tzadok regarding the permitted measure.