1) HEATING AN EGG IN SAND HEATED BY THE SUN
QUESTION: In the Mishnah (38b), the Tana Kama forbids one to heat an egg by breaking it over a cloth that was heated by the sun. Rebbi Yosi argues and permits it. He agrees, however, that one may not heat an egg by placing it into sand that was heated by the sun. The Gemara asks why Rebbi Yosi agrees in the latter case, if he maintains that one is permitted to do Hatmanah in material that was heated by the sun.
Rabah answers that Rebbi Yosi prohibits heating an egg in sun-heated sand because of a decree that one might think that it is also permissible to do Hatmanah in ashes in which there are coals, which one might then stoke in order to increase their heat.
Rav Yosef answers that Rebbi Yosi prohibits heating an egg in sun-heated sand because one might move dirt in order to bury the egg in the hot sand.
The Gemara makes it clear that the opinion that says that the decree was because of the fear that one might move dirt is not concerned that one might come to do Hatmanah in ashes.
(a) Why is that opinion not concerned that one might do Hatmanah in ashes? One who places an egg into a material that adds heat transgresses a Gezeirah that the Rabanan made lest one do Hatmanah in ashes with coal and stoke the coal, as the Gemara clearly says (34b)!
(b) Furthermore, even if sand would be a material that does not add heat, the Halachah is that Hatmanah in material that does not add heat is forbidden on Shabbos itself (see Insights to Shabbos 34:1)!
(a) TOSFOS (DH Ela) answers that the Gezeirah against Hatmanah in a material that adds heat (lest one come to do Hatmanah in hot ashes and then, on Shabbos, stoke the coals) applies only on Friday, before Shabbos. This Gezeirah does not apply on Shabbos, because there is no fear that one will do Hatmanah in hot ashes on Shabbos and thereby cook uncooked food.
(b) Tosfos answers the second question by pointing out that the Gezeirah not to do Hatmanah on Shabbos lest one attempt to bring the food to a boil applies only to something that is already cooked. That is, normally one insulates a food that is already cooked in order to preserve its heat; such Hatmanah is prohibited lest he unexpectedly find that his food is cold and heat it up, forgetting that it is Shabbos. In contrast, in the case of the egg, the person not only knows that his food is cold, he knows that it is not even cooked. We are not concerned that he will attempt to cook it from scratch (using fire) on Shabbos.
2) DOING "HATMANAH" IN MATERIAL THAT ADDS HEAT
QUESTION: Rav Chisda says that we learn from the incident with the people of Tiberias that Hatmanah in material that adds heat is prohibited. Why does he say that we learn this from the incident with the people of Tiberias? The Mishnah later (47b) states this Halachah explicitly!
(a) TOSFOS (DH mi'Ma'aseh) answers that without the incident in Tiberias, we might have thought that the Mishnah later only prohibits Hatmanah during Bein ha'Shemashos, but while it is still daytime on Friday afternoon Hatmanah is permitted. We learn from the incident in Tiberias that even on Friday before Shabbos, Hatmanah in material that adds heat is prohibited.
(b) Tosfos suggests another answer. If we had only the Mishnah later, we might have thought that Hatmanah is prohibited only l'Chatchilah, but b'Di'eved if one did Hatmanah the food is permitted. The incident in Tiberias teaches us that the food itself is also prohibited.
(c) RABEINU YONAH (cited by Tosfos) answers that the incident in Tiberias teaches that one may not do Hatmanah in material that adds heat even with a raw food. Even though one is permitted to leave a raw food on the stove (Shehiyah) on Friday, one may not do Hatmanah with a raw food.
(d) The RITVA, RAMBAN, and RASHBA explain that the Halachah stated in the Mishnah later (47b) was taught as a result of the incident in Tiberias. It was in Tiberias, during the episode described in the Mishnah here (38b), that the Chachamim first said that Hatmanah in material that adds heat is prohibited.