Déjà vu (Deja VU) describes the strange experience of a situation feeling much more familiar than it should. 

It is a French loanword expressing a feeling that one has lived through the present situation before. Young people experience déjà vu the most. 

Although some interpret Déjà vu in a paranormal context, mainstream scientific approaches reject the explanation of Déjà vu as "precognition" or "prophecy". It is an anomaly of memory whereby, despite the strong sense of recollection, the time, place, and practical context of the "previous" experience are uncertain or believed to be impossible. 

How common is Déjà vu?
The percentage of people who experience Déjà vu is probably somewhere between 30% and 100%. No one can be sure about the percentage as different people experience Déjà vu in different phases of life.

Who experiences Déjà vu the most and what does this tell us?
Young people experience Déjà vu the most. Having said this, depending on how old are you, you may have to wait a while until you have your first Déjà vu experience. A very small number of people say they had their first Déjà vu by the age of 6. More people report their first Déjà vu experiences as having happened sometime before they were 10 years old.

The reason it may take a while to have your first Déjà vu experience is that you need to be able to work out whether the feeling of familiarity you have is stronger than it should be. For many younger kids, this may be a tricky thing to do.

By the time you reach an age between 15 and 25, you will probably be having Déjà vu experiences more often than you will ever have them after that. The number of Déjà vu experiences people report steadily decreases after 25. This is puzzling for researchers because we are used to thinking of memory problems increasing with age, and not decreasing with age.

This may tell us something important about Déjà vu it is not a memory problem at all. It is a sign of a healthy mind that can spot incorrect familiarity signals. Perhaps what is happening in people over the age of 25 is that they get worse at spotting incorrect familiarity signals and start believing them.

What causes Déjà vu?
This is a really important question, but it is also still a mystery. We can get some clues from groups of people who report more Déjà vu than most. One of these groups contains people who have a condition called "temporal lobe epilepsy". 

Epilepsy causes brain cells to send out-of-control electrical signals that affect all the brain cells around them, and sometimes even all the cells in the whole brain. These signals can move through cells in the brain like dominoes, each one knocking over the ones that are next to them. This is called a "seizure" and can result in people with epilepsy briefly losing control of their thoughts or their movements. 

In people with temporal lobe epilepsy, we know that seizures start in the temporal lobe. This is a part of the brain just inside from the top of your ears, and it is important for making and remembering memories.

Importantly for Déjà vu researchers, people with temporal lobe epilepsy often report having Déjà vu just before they have a seizure. This tells us that Déjà vu is probably linked to the temporal lobe of the brain. 

Conclusion;
Déjà vu is an interesting and unusual experience where something feels very familiar, but we know it should not feel as familiar as it does. 

This experience is important because it shows us that remembering happens with a series of steps, some of which can go wrong. 

Many people also believe that Déjà vu can occur because the time dimensions wrangle with each other and sometimes our parallel universes may get entangled. Whatever it may be, it is an out-of-the-earth experience.