MYTHOLOGICAL ANECDOTES ABOUT BIRTH OF GANESHA ! 
The most well-known story is probably the one taken from the Shiva Purana. Once, while his mother Parvati wanted to take a bath, there were no attendants around to guard her and stop anyone from accidentally entering the house. Hence she created an image of a boy out of turmeric paste which she prepared to cleanse her body (turmeric was us

ed for its antiseptic and cooling properties), and infused life into it, and thus Ganesha was born. Parvati ordered Ganesha not to allow anyone to enter the house, and Ganesha obediently followed his mother's orders. After a while Shiva returned from outside, and as he tried to enter the house, Ganesha stopped him. Shiva was infuriated at this strange little boy who dared to challenge him. He told Ganesha that he was Parvati's husband, and demanded that Ganesha let him go in. But Ganesha would not hear any person's word other than his dear mother's. Shiva lost his patience and had a fierce battle with Ganesha. At last he severed Ganesha's head with his trishula. When Parvati came out and saw her son's lifeless body, she was very angry and sad. She demanded that Shiva restore Ganesha's life at once.
Unfortunately, Shiva's trishula was so powerful that it had hurled Ganesha's head very far off. All attempts to find the head were in vain. As a last resort, Shiva approached Brahma who suggested that he replace Ganesha's head with the first living being that came his way which lay with its head facing north. Shiva then sent his celestial armies (Gana) to find and take the head of whatever creature they happened to find asleep with its head facing north. They found a dying elephant which slept in this manner, and after its death took its head, attaching the elephant's head to Ganesha's body and bringing him back to life. From then on, he was called Ganapathi, head of the celestial armies, and was to be worshipped by everyone before beginning any activity.
[Material source :Shiv Puran].