Once there was a demon called Gajamugasuran. He was a great devotee of Shiva and pleased with his presence, Shiva granted him some boons. Gajamugasuran became very proud and powerful and he started troubling the gods. Tired of his antics, the gods asked Shiva to save them from Gajamugasaran. Shiva sent Ganesha to help the gods. Ganesha went with several weapons such as bow and  arrow, sword, and even an axe to destroy Gajamugasuran. But the demon had been given boon that none of these weapons could cause him harm, so Ganesha's attempts to destroy the demon proved to be ineffective. Finally, Gajamugasuran took the form of a small mouse and rushed towards Ganesha. Ganesha was very clever and immediately sat on the little mouse.  Gajamugasuran was defeated and was forced to apologise for his deeds. Since then, Ganesha is always accompanied by a mouse as his vehicle.