Biz Stone announced he's leaving Twitter and starting up his old start-up he had started before starting Twitter. Startling! Stone says he'll be available when Twitter needs him, but he's leaving the day to day business stuff up to the other Twitter higher-ups. Stone, along with Twitter co-founder Ev Williams and ex-Twitter product chief, Jason Goldman, will relaunch Obvious, the incubator that spawned Blogger and Twitter. It's not clear what Obvious will do exactly (this sentence sponsored by the American Irony Foundation). According to Stone: "The Obvious Corporation develops systems that help people work together to improve the world."