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Cooking with direct sun heat: To cook with the [heat of the] sun directly, such as to leave an egg in the sun in order to cook, or to leave water in the sun in order to heat up is allowed because doing so is not the common form of cooking [and thus is not Biblically forbidden]. [Furthermore, this is even Rabbinically permitted as the Sages] did not decree [against cooking] with the sun due to [that one may then also come to cook] with fire being that [cooking in the heat of] the sun will not be confused with cooking with fire.
Summary-The following form of cooking is completely permitted:
To cook, bake, or do any of the above-mentioned activities through the direct rays of the sun, [although regarding melting fat see below].
May one use a magnifying glass to cook his food?
Is water heated in the solar boilers [Dud Shemesh] considered Chama or Toldos Chama?
Some Poskim rule that the heating of water in the solar boilers is considered Toledes Chama and is hence Rabbinically forbidden. Others rule it is considered Chama and is hence permitted to be heated. According to all the water itself is considered Toledes Chama and is hence forbidden to cook items within it.
 Admur 318:7; Michaber 318:3
 Halacha 7
 Kaf Hachaim 318:46 [regarding making a flame with the glass]; See Maor Hashabbos 1:17 footnote 8 [p. 284]
 Lev Chaim 3:68
 Shaar Zekeinim 2 p. 86
 Shevisas Hashabbos Mivashel 44
 The reason: As perhaps this is considered Chamah, or perhaps it is considered Toldas Chamah, hence through a gentile one may be lenient in a time of illness.
 Minchas Yitzchak 4:44; Shevet Halevy 1:94; Bris Olam Ofeh 13
 Tzitz Eliezer 7:19; Yabia Omer 4:34; Har Tzevi 188; SSH”K 1:45 simply writes “It is proper…”