Justin Bieber appeared on the Late Show last night and taught ol' Dave a little bit about Twitter. It's a little alarming that Letterman doesn't know what an "underscore" is but he gets a pass on this one anyway.

Who doesn't get a pass, though? Bieber. When asked what grade he's in at high school, he responded "I don't know, I'm Canadian." Cue the shot of fellow-canuck Paul Shaffer!