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For melakha purposes, cooking means preparing or improving food by using heat. The underlying principle of Shabbat law is that one may not create a new entity. Therefore, the prohibited type of cooking is taking raw food and turning it into cooked food. Included in the prohibition is taking something partially cooked and finishing the job. However, if food is already fully cooked, reheating it is not prohibited, as nothing new is being created. At what point is a dish considered to be fully cooked? From the point at which it could be served to guests without the host feeling compelled to apologize. Heating water is considered cooking as well.
