The Gemara in Shabbos teaches that there is a גזירת מרחצאות - one may not bathe in hot water that was heated even before Shabbos because the bathhouse owners began to heat water on Shabbos (to generate business), claiming that the water had been heated before Shabbos. In order to stop this practice, the Chachomim prohibited bathing in hot water on Shabbos [Shabbos 40a].
There is an argument about rinsing [i.e. showering] with hot water that was heated before Shabbos. R' Yehuda forbids this while R' Shimon permits this. Then the Gemara says:
"מחלוקת בכלי אבל בקרקע דברי הכל מותר"
The argument pertains to water that is in a כלי but water that is in the ground is permitted according to all.
What is the rationale for this distinction?
Rashi writes:
בכלי - דהרואה אומר היום הוחמו ומוכחא מילתא דתולדות האור נינהו ואתי להחם בשבת כגון נותן צונן בחמין:
אבל בקרקע - כגון אמבטאות של מרחצאות והן חמי האור:
מותר - דבטלי להו מתורת חמין:
If the water is in a כלי then the person who sees this will assume that it was heated on Shabbos [because water doesn't remain hot for long in a כלי] and in addition it is clear that it was heated with a derivative of fire and he will come to heat water on Shabbos by, for example, adding cold water to hot water. But water that was in the ground that was heated by fire before Shabbos is permitted to rinse with. Why? Says Rashi דבטלו להו מתורת חמין - they were nullified from having the status of hot water.
Three questions.
First, what is the pashut pshat [a Chinese term they often use when studying Bible - 以 个] in the last Rashi? Why is water in the ground בטלו מתורת חמין???
Also, Rashi, although he never saw a baseball game in HIS LIFE [and he got through it unharmed!!] came right out of LEFT FIELD!! He says דהרואה אומר היום הוחמו - one who sees this will assume that the water was heated on Shabbos and he will do the same. But that idea appears NOWHERE in the Gemara. The reason given is because of the errant behavior of the bathhouse owners that we mentioned previously. So why did Rashi seemingly invent a new reason?
Last - but not least: Rashi says that he will heat the water by adding cold to hot. Why doesn't he say גלייך - straight and simple. He will heat water by placing it next to fire!!?
Here is a shiur on this topic [that doesn't address these issues].
