Deja Vu are actually french words and mean "already seen". It's this weird feeling, this sensation when you experience a moment and you are sure that you have experience it before, exactly the same. If you have ever felt it you' ll know how strange and sometimes unnatural it feels. It's like you go back a movie for a split second and watch a moment again, only this happens with your life. There are many theories and beliefs, some logical others a bit lunatic, why and how this happens. The one that is believed to be true among scientists has to do with how our eyes, nerves and brain work.
Whenever our eyes see something, they both sent a message to the brain via the nerves. The part of the brain which is repsonsible for seeing receives these two messages the
exact same moment. I don't remember whether the eyes sent the message simultaneously or the nervous system is responsible for synchronizing em. What happens in case of Deja Vu? A little malfuction happens and the first message is sent just a little bit faster than the second one. So, when the brain receives the first one, it considers it a valid signal sent from the eyes and proceeds to decode it in order to happen this that we call vision or simply seeing. But the second one arrives split seconds later and the brain is confused since it has a second "seeing" mesage timed from the exact same moment. So it rejects the second one and make us believe that it comes from the memory. This causes all the confusion of Deja Vu.
Note that all the above happen extremely fast, our brain is literally being bombarded by titanic amounts of information every second, so it processes them extremely fast as well. And it can't be prooved since we aren't even close in understanding this marvelous, organic super-computer known as human brain. just a theory, which is based on some bilogical facts.
And the other one tomorrow cause my brain processed too much data for today and needs some rest