Ganesha is formless, but on Ganesh Chaturthi we make a form and invoke him by saying, ‘Ganapati, though you are everywhere, I would like to offer my gratitude to you by offering sweets, by offering all that you offer to me; flowers & fruits. You make the sun & moon go around. I’ll take a little camphor around you. So all that you gave me, I offer it back to you!’ And in the end you say, ‘you have come from my heart and you will get back into my heart,’ and leave the idol in the water.
Happy Ganesh Chaturthi!