Justin Beiber appeared on "The Late Show With David letterman" last night and schooled the host in the ways of Twitter. Bieber whipped out a smart phone and asked if he could send a tweet from the guest chair. Letterman agreed. "Late underscore show?" Bieber asked a clueless Letterman. "It’s the Late Show. I’m Dave," the host said to laughs. Bieber told Letterman he tweeted: "I’m just hanging out with my boy Dave." Watch the clip.