I have the sense I have already posted this...
Déjà vu is the feeling that we have already done something before. There have been many hypotheses about why this occurs, including appeals to parapsychological explanations, but recent research is now showing that this is really an experience of memory failure. It is a "glitch" where we somehow connect a present experience to a real memory from the past, without knowing we have that memory. So, you might think you have been in a new neighbor's house before, even though you just moved to town, but it is really because the house has a layout similar to another house (or another building of some kind) that you have been in, but do not remember or are not connecting to the present experience. Anne Cleary has done excellent work in this area, and can now pretty reliably create the Déjà vu experience in lab participants, by giving them related experiences that they do not necessarily feel were related (but that are close enough to evoke the sense of Déjà vu).