Why is There a "B" in "Doubt"?

In this three-minute video, Gina Gooke explains why the English word "doubt" has a silent "b" in it. The short answer is "because of Latin," but the longer answer is much more interesting, touching on the migration of words from Latin through French and into English -- and the surprising Old English word for doubt that we had before "doubt" made us think twice. This is worth a look for word nerds, students, and anyone who has wondered why "doubt" is spelled weirdly.
This is one of TED Ed's new mini-lessons. There's a site for the video containing quizzes and stuff. Neat. Also relevant: Cooke's video making sense of spelling.