It might just be the weirdest experience you'll ever have sober, but what exactly is deja vu?

One thing it definitely is, is common.

Two-thirds of us have had it, with younger people, globetrotters, and film fans likely to get it more frequently.

Because of its inherent weirdness, deja vu was long thought of alongside paranormal events like clairvoyance and reincarnation.

In fact, it was parapsychologist Émile Boirac who first named the feeling in the 1870s using the French for "already seen."

The focus on the uncanny has persisted, and in films like "The Matrix," deja vu is a glitch in the computer simulation.

So, what's actually going on?

The truth is, no one is 100 percent sure, but psychologists have suggested dozens of possibilities combining theories of memory, perception, and cognition.

One is divided perception.

Maybe our brains process a situation in a quick and shallow way before we become fully aware of it, and then we get a jolt of having seen it before.

Another is dual processing.

Incoming signals enter the temporal lobe from both hemispheres of the brain, one a millisecond later than the other.

And it's in this moment of delay that deja vu occurs.

Others speculate that errors around the hippocampus—the brain's librarian—are to blame.

The problem with studying deja vu is that neurologists can't very well wait around for it to happen.

One solution has been to look at people with temporal lobe damage.

Many find that they get chronic deja vu.

Another way to study deja vu is to induce it under lab conditions.

In 2012, one study used virtual reality to immerse people in different 3D environments, some of which were very similar in layout.

For instance, a doctor's waiting room and an aquarium, with furniture arranged in the same configuration.

People were more likely to report deja vu when they encountered environments that had a similar layout to previous, forgotten scenes, suggesting it's a memory phenomenon.

A 2014 study had very different results.

Those who took part were shown a series of words with a secret common theme, words like bed, pillow, nap, dream.

The linking word, "sleep" never appeared.

Viewers were asked to keep note of any words beginning with S.

Those who took part were later asked if any words began with S, and sure enough, they said no.

But many also felt that they had been shown the word "sleep."

For two thirds of people, this confusion was tantamount to deja vu.

Neurologists have used this method to scan the brain during deja vu.

They found that rather than being a memory error in the hippocampus, deja vu involved the frontal areas of the brain responsible for decision making.

This led some to suggest that deja vu is a sign your brain's memory-checking mechanisms are actually working well.

But, if you're looking for something a bit more out there to explain deja vu, try quantum entanglement.

Perhaps through the mysterious affinity of subatomic particles, deja vu might actually be a window into a parallel universe, or else a blip in time.

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