Every part of Ganesha's form teaches a lesson for life. The large head stands for thinking big, the small eyes for concentration, the big ears for listening more and the small mouth for speaking less. The large belly shows the strength to digest all of life's joys and sorrows.
The little mouse stands for the restless mind and its desires. Ganesha rides upon it, meaning the mind is mastered through wisdom. The modak in his hand symbolises the joy of knowing the Self.
The Ganesh festival is celebrated for ten days, from Bhadarva Sud Choth to Anant Chaturdashi. In 1893 Lokmanya Tilak made it a public festival and used it to bring society together. The immersion of the idol teaches us that creation and dissolution are both part of life: "Ganpati Bappa Morya, pudhchya varshi lavkar ya!" (O Lord Ganpati, come again soon next year!)