G-d did not create anything new on Shabbat; the 7th day of rest. All of creation was completed in the first 6 days of creation. Since then G-d has been maintaining the world, but not creating anything new. To commemorate this, Judaism defines 39 categories of creative work which are forbidden on Shabbat. E.g.: baking, planting and lighting fires. See http://www.answers.com/topic/39-categories-of-activity-prohibited-on-shabbat for an introduction to these 39 categories.

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We're not permitted to work on the Shabbat (Exodus ch.20). This includes 39 categories (Talmud, Shabbat 73b) of productive interaction with the world, such as planting, writing, building, kindling fire, using electricity, etc.

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