Well, Whadya Know

What's the purpose of that snap-box contraption they clack in front of a movie camera just before filming a take?

The purpose of this mysterious ritual is to synchronize the soundtrack with the picture. In the editing room during post-production, the editor can align the soundtrack for the entire scene by synchronizing the clack sound with the frame in which the box - it's called a clapboard - is snapped closed.

[source: The Straight Dope by Cecil Adams]

Why do we say that someone who is kept in isolation is in "quarantine?"

The word quarantine originated in the Middle Ages with the most famous epidemic in Western history, the black plague. Forty days appeared to be the incubation period for this dread disease and in Italy, they simply called this period, in which you were kept away from everyone, the "quarantina," Italian for the number forty.

[source: A Browser's Dictionary by John Ciardi]

Why do we call a computer problem a glitch?

The word glitch is relatively new, a product of the space age and the era of advanced electronics. It comes from the German "glitschen," and via the Yiddish, "glitshen." Both mean, "to slip."

[source: The Secret Lives of Words by Paul West]