Google homepage in the US displayed an interactive doodle, that animates some particles which;re used to create the Google logo. It's very likely that the doodle celebrates Google's 12th birthday, which Google celebrates generally on Sep 7th or Sep 27th, depending on the year. "Google opened its doors in Sep 1998. The exact date when we celebrate our birthday has moved around over the years, depending on when people feel like having cake," said Google.
Happy Birthday, Google!
Here're some previous Google birthday doodles: