Why is the idol of Lord Ganesha immersed in water (visarjana)?
Every year Clay statues of Lord Ganesha are immersed into the water bodies or in sea. Gradually the clay dissolves into water. Ganesha's image disappears. This ritual is designed by our ancestors to draw attention to the 'ephemeral nature of life and worldly things'. Showing that nothing in this mortal world lasts forever. It celebrates the fact of life of Creation(Idol construction),Perpetuation and finally Visarjan or immersion(Destruction) in water bodies with fun and fair, as celebration pointing to the fact and Showing that death is imperative/essential for creation of new that's why people say
"गणपति बब्बा मोरिया अगले वरस तू जल्दी आ !".
Source : wiki.answers.com
However, many people who keeps a Ganesh in their homes choose to perform the immersion much before this. There are no hard and fast rules as to when it's done. Rather it's up to personal preference.
Source : Ganesh Visarjan
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