Shabbos, Chapter 22, Mishnah 4. The Mishnah opens by stating: "Nosnin tavshil l'toch ha-bor bishvil sh'yehei shamur" - one may place a cooked dish inside a pit on Shabbos so that it will be preserved there, meaning that it should stay cold and not warm up.
The novelty of the Mishnah:
The Gemara explains that the novelty of the Mishnah is that we do not suspect that while setting the pot down inside the pit a person will come to smooth out the floor of the pit, so that the pot should stand properly and not tilt to one side or the other. Such smoothing is an improvement of the ground, and it falls under the prohibition of plowing. The Mishnah teaches that there is no need to be concerned about this.
Good water in bad water, and cold water in the sun:
We further learned that one may place good water - water that is fit for drinking - inside bad water that is not fit for drinking and is colder, so that it should cool the drinking water. At first glance there is nothing novel about this permission, and it is taught here mainly because of the halachah that follows it.
The next halachah deals with placing cold water so that it should warm up, and there are two versions of the text:
"V'es ha-tzonen ba-chamah" - placing cold water in the sun, so that it should warm up.
"V'es ha-tzonen b'chamin" - some have the version that he places it inside hot water, so that it should warm up.
Either way, the novelty is that we do not suspect that he will come to place the cold water on top of glowing coals. And since this permission was taught, the opposite permission was taught before it as well - to place the drinking water inside the cold water that is unfit for drinking so that it should cool off - even though there is nothing novel about it.
One whose clothes fell into water on the road:
The Mishnah continues: "Mi she'nashru kelav ba'derech ba'mayim - mehalech bahen v'eino chosheish". One whose clothes became soaked with water while he was traveling on the road may continue walking in them, even though they are very wet. Two explanations are given for this permission:
Because of maris ayin: There is no concern that onlookers will say that he laundered his clothes on Shabbos.
According to the Rambam: There is no concern that he will come to wring out the clothes.
However, once he has left the road "v'higia la'chatzer ha'chitzonah" - and reached the outer courtyard of a village or a city, a secure place where one can leave objects and they will remain there - he may spread the wet garment out in the sun so that it should dry, "aval lo k'neged ha'am" - but not in a place where the public sees him.
The halachah:
The Gemara teaches that we do not rule in accordance with this Mishnah. For we hold that anything that involves maris ayin - and here it appears as though he laundered his clothes and hung them up to dry, which is forbidden on Shabbos - is forbidden even when not in a public domain, and even in the innermost chambers and in private. It emerges that the permission to spread out the clothes to dry provided that it is not in front of the public is not the halachah; rather, it is forbidden to do so even in private.