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There was an asura called Gajamugasuran.He did deep penance and worshipped Lord Siva,who granted him several boons to him by which he became extremely powerful.He put the Devas and rishis to great trouble. He forced the Devas to do 1008 punishes
(Thoeppukaranams) in the morning,1008 in afternoon and 1008 in the evening.Thoepukaranam means to sit down and then stand up holding one's ears with crossed hands,left ear with right hand & right ear with left hand.The Devas prayed to Lord Shiva to rescue them from this evil asura.
Lord Shiva sent Ganesha to overcome Gajamugasuran.Ganesha went with several weapons such as bow arrow,sword,axe,etc.However, since Gajamugasuran had been granted the boon that he be destroyed by all these, none of these weapons used by Lord Ganesha were effective.Vinayaka broke off his right tusk and used it to kill Gajamugasuran.
Gajamuga still rushed at Vinayaka in the form of a mooshikam ie.a small mouse. Ganesha crushed the ego and vanity of Gajamugasuran and sat on the mooshikam.The humbled Gajamugasuran bowed before Ganesha,who then accepted the mooshikam as his "Vahana".
Lord Ganesha is the source of everlasting wisdom.He crushed ego and vanity and reduced it to A size of a mooshikam and used it as his vehicle "Vahana"
The Lord Ganesha can be pleased by doing "Thepukaranam".