What causes deja vu?

Déjà vu. French for 'already seen' is the sensation of having seen something/person/place before even though you know you haven't. Theories as to the cause abound [see following site for some: http://science.Howstuffworks.Com/science-vs-myth/extrasensory-perceptions/deja-vu.Htm] but no one knows for sure.