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Even though reheating food is permitted on Shabbat, a person may not do so over an open fire or burner, because it appears to be cooking. (We are concerned people will mistakenly conclude either that this person is transgressing the prohibition of cooking, or that cooking on Shabbat is permissible.) Additionally, there is a concern that someone reheating this way might end up adjusting the flame, thus transgressing the prohibition of lighting a fire. Therefore, the best way to reheat cooked food on Shabbat is by using a plata, a warming tray with a hidden heating element and a fixed, non-adjustable temperature.
