Why Durva Grass is offered to Lord Ganesha?

Durva grass is a three-bladed plant known to attract immense energy of ganesh. once while fighting a demon ganesha ended up swallowing him. This gave him a strong burning sensation in his stomach. When all else failed to provide relief, a group of sages offered 21 blades of the durva grass. It was after eating them that ganesh felt some relief. Since then, it is said to be one of his favourite food items.