How did Ganesha lose his tusk?

the tusk of lord ganesha got broken for the sake of writing mahabharatha and according to Hindu mythology veda vyas started to write the greatest epic of Hinduism mahabharatha and after some time he felt exhausted and so he called lord ganesha and as ganesha came forward to help veda vyas ,lord ganesha started writing mahabharatha as veda vyas narrated and remember veda vyas also had lived along with characters of mahabharatha.