You must have read tales known as the Jataka Tales. The Jatakas are the tales about the former lives of the Buddha. Before he attained enlightenment, Buddha had been born as Bodhisattva. It is said that he was born again and again 547 times, sometimes in a human form, and sometimes as a bird or beast or fish.
The Jataka deals with these lives of Buddha. These tales always begin with an incident in the life of the Buddha, usually a sermon he is giving, which he illustrates with a story from one of his previous lives. The Jatakas are primarily moral tales, illustrating the wisdom and goodness of the Bodhisattva figure.
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