Google is very creative ! Creativity extends even upto the logos of Google. To commemorate a very special event Google uses a cutomised and artistic logo. If you happen to visit Google.com on that day you can see that special logo. This customised logos built to commemorate a special event is often called as Doodle.
For eg : Last year (2006) on Louis Braille's ( The man who created Braille - The print language of the blind !. He is the one who brought light into the minds of millions of blind people worldwide ) Birthday on January 4 Google was written in braille.
Google uses such logos to grab the attention of its users to the special event. As google.com is a neat and clean website a minute change is easily noted.
To see the logos for yourself vist the following places ! You can see logos that they have used, right from their start in 1999 !
Official logos : http://www.google.com/stickers.html
Fan Logos : http://www.google.com/customlogos.html
Holiday Logos : http://www.google.com/holidaylogos.html
Have a nice time
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