Deja vu, a term that most of us have started using often after knowing what that means.
A feeling that you've experienced something before. A feeling which makes you feel as if you have been in that situation before when you definitely know that it's the first time it's happening for real. Yet that feeling refuses to go away.

Some people say that that feeling is from their previous birth or something like that.

It's hard to believe it, but why does it happen then?