The Shulchan Aruch (Orach Chaim 260) writes that there is a mitzvah to wash one's body on Erev Shabbos with hot water, as seen in Tractate Shabbos 25b. The Biur Halacha writes that one can argue about using warm water, but cold water is definitely no good, and one will not have fulfilled the mitzvah.
Why is this so? Shouldn't it depend on the situation? If it's hot outside, perhaps I don't want to take a hot shower, and by taking a hot shower, I will feel uncomfortable on Shabbos!
What is considered hot, for this purpose? Surely, it cannot mean that the water has to be so hot that one gets burned because of it.