Gajasua (an asura or demon who has taken the form of gaja or elephant) when started misuing the boons of shiva to cause harm to the world shiva takes his head off but gajasura being a staunch devotee of shiva asks one last favor to be remembered forever and stay near him all the time. Shiva thus removes him from the cycle of birth and death.
Here in kailash parvathi has created a boy named vinayak who was standing outside as a guard when shiva's wife parvathi is bathing and when shiva tries to get inside vinayak prevents him. Shiva was also cursed by one of his own devoted rishis to be responsible for his own son's demise and shiva himself has given the rishi the power to curse anyone and thus by not fullfulling his curse he wouldnt want to go back on his word. So shiva cuts vinayak's head and replaces it with the gajasura's elephant head.
source: http://www.harekrsna.com/philosophy/associates/demons/siva/gajasura.htm
Here in kailash parvathi has created a boy named vinayak who was standing outside as a guard when shiva's wife parvathi is bathing and when shiva tries to get inside vinayak prevents him. Shiva was also cursed by one of his own devoted rishis to be responsible for his own son's demise and shiva himself has given the rishi the power to curse anyone and thus by not fullfulling his curse he wouldnt want to go back on his word. So shiva cuts vinayak's head and replaces it with the gajasura's elephant head.
source: http://www.harekrsna.com/philosophy/associates/demons/siva/gajasura.htm
