Kauravas were not 100 they were actually 101.
100 brothers and 1sister
The story goes like this:-
Queen Gandhari : once, sage vyasa came to visit gandhari in hastinapur. Indeed, gandhari took great care of the comforts of the saint and tried to ensure that he had pleasent stay in hastinapur. The saint pleased with gandhari, granted her a boon.
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Gandhari expressed her wish for a 100 sons who could as powerful as her husband. Vyasa, in turn granted her the boon and in due course of time, gandhari was pregnent. However,even though two years had passed, still the baby remained unborn.
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Then, one day, gandhari gave birth to a hard piece of lifeless flesh that was not a baby at all. Gandhari was devastated as she had expected a hundred sons according to the blessing of rishi vyas.
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.Gandhari was about to throw away the piece of flesh. Ved vyas appeared and told her that his blessings could not have been in vain. He asked gandhari to arrange for 100 jars to be filled with ghee.
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Before the process could start gandhari pressed her desire for having a daughter. Ved vyas agreed and then chopped the piece of flesh into 101 pieces and placed them in the jars, promising them that they would develop into the 100 sons and 1 daughter she desired.