One must not place an egg at the side of a boiler for it to be roasted, and one must not break it into a [hot] cloth; but Rabbi Yose permits it. And one may not put it in [hot] sand or road dust for it to be roasted (Shabbat 38b Mishnah).
Last time, we mentioned that our Sages inherited prohibitions on a number of activities that are permitted by the Torah, but not in consonance with the spirit of Shabbat. Our Sages knew that prohibiting all everyday activities on Shabbat would not only be impossible, but also make Shabbat overly burdensome. Shabbat is a day of sanctified rest as an offering to Heaven, but it is also a day of earthly pleasures. As a result, the Sages limited these protective "Rabbinic prohibitions" on Shabbat to a small number of categories. We find in the above text an example of one such category: it is a Rabbinic prohibition to perform acts that could easily be confused with Torah prohibitions.
As we saw in mishnah Shabbat 7:2, cooking on Shabbat is a Torah prohibition. What is cooking? Is it merely the application of heat to food to make it edible? Our Sages answered, no. Human will and human behavior dictate the conventional definitions of all of the prohibited acts. There is a way that people cook, and a way that they generally do not cook. In the time of the Mishnah and the Talmud, cooking generally involved fire. The cases in our mishnah above do not. Laying an egg down on the pavement for it to fry is not a case of conventional cooking. As such, it cannot be included in the Torah prohibition on cooking on Shabbat. Nonetheless, our Sages say, preparing an egg in such a way on Shabbat would lead to a kind of confusion that would ultimately be destructive to the spiritual discipline Shabbat represents. We must prepare our food in advance to truly taste the Shabbat spice, a taste of the world to come.