Google often changes its logo on special holidays. On St. Patrick's day they’ll have something Irish, on New Year's Eve/Day, it'll show some fireworks, and so on and so forth. Holidays aside, the company also displays these Google Doodles on days that mark important moments in history. Take the August 25 logo, which marked the 400th Anniversary of Galileo’s Telescope, or the August 12 logo celebrating the annual Perseid meteor shower (both shown below along with the weekend's mystery Doodle).
However Saturday's Google Doodle has people scratching their heads. A UFO? But why? No explanation for the picture was offered other than a statement from the company that said, "We consider the second 'o' critical to user recognition of our brand and pronunciation of our name. We are actively looking into the mysterious tweet that has appeared on the Google twitter stream and the disappearance of the 'o' on the Google home page. We hope to have an update in the coming weeks."
Later in the day, a coded tweet from Google solved the mystery. Once deciphered, the message revealed that the Doodle was celebrating the 20th anniversary of Zero Wing, the poorly translated Japanese video game that birthed the internet meme, "All your base are belong to us." By substituting each number for its corresponding letter in the alphabet, the message reads "all your O are belong to us."
Btw, try googling for "recursion" ;)