What deja vu is and what causes it is a mystery.
That distinct illusion of having been there and done that has no explanation. The parapsychologist will tell you it's a past life experience. Yogi Berra will remind you it seems like you've felt it before. And most scientists will throw their hands up.
Some believe déjà vu involves emotional responses to similar events; others figure the brain short circuits, sending an event to memory just a split second before putting it into consciousness.
Its fleeting nature makes déjà vu about as easy to study as the afterlife. Some people have a chronic variety, though, and so one study is attempting to get inside their minds. Chris Moulin, who runs a memory clinic at the University of Leeds in the UK and is doing the research, describes one patient who illustrates how déjà vu might be related to memories being mixed up by the brain: "When this particular patient's wife asked what was going to happen next on a TV program he'd claimed to have already seen, he said, 'How should I know? I have a memory problem!'"