Google doodle are more frequent this year when compared to previous years.Google celebrates every occasion with a new doodle which nowadays become more interactive rather than static images used before,now the interactive doodles are created using html5 and css3 technique.
Google doodle for earthday 2011 was as usual interactive and I analyse how google guys have done it and here is a simple explanation on how it was done.
as you can see several images are tied to one single images which form css sprites which help in reducing the page load time by reducing the image requests.
2.The moving image
The entire logo is sliced into different regions having separate hover events
see this video to understand how the css technique was used to make the interactive doodle.