One day Goddess Parvati was at home on Kailash (Shiva's abode) preparing for a bath. She told Nandi (Shiva's Bull) to guard the door and let no one enter in. Nandi faithfully guarded the door. But, after Shiva's arrival to Kailash, being himself loyal to Shiva first, Nandi had to let him pass.
After this incident, Parvati wanted someone as loyal to Herself as Nandi was to Shiva. The next time, while taking her bath, in the desire of owning a loyal son, Parvati rubbed off the dust, oil, sandalwood, turmeric paste from her body and created the figure of a young boy using this material. She breath life into this figure, whom she called Ganesha, declaring him to be her own loyal son. She asked him to guard the door and let no one enter while she was bathing.
After this incident, Parvati wanted someone as loyal to Herself as Nandi was to Shiva. The next time, while taking her bath, in the desire of owning a loyal son, Parvati rubbed off the dust, oil, sandalwood, turmeric paste from her body and created the figure of a young boy using this material. She breath life into this figure, whom she called Ganesha, declaring him to be her own loyal son. She asked him to guard the door and let no one enter while she was bathing.
A little later Shiva happened to come there. Ganesha stopped him at the door. Shiva had never seen the boy and was angry when he stopped him enter his own abode. He tried to ignore him and trying to brush him aside took a step. The boy fought with Shiva using his weapons. During the fight Shiva with one blow beheaded Ganesha.
Parvati was full of grief when she heard of her sons death. Now Shiva had to revive him. Parvati asked him to get the head of anyone he sees first sleeping in the North direction. Shiva found the head of an elephant and revived the boy. Now Parvati was only partially appeased. She asked for more for the boy from Shiva. He offered Ganesha a boon that he will be worshiped first before all Gods. Also he will be called Vighneshwara (remover of all obstacles).
