About two-thirds of all people experience déjà vu at least once in their lives. If you never have, this is what it’s like: Déjà vu is the acute, momentary sensation of having already experienced whatever it is you’re currently doing or seeing. You know that logically this is not possible. Yet another part of your head — you might say a gut feeling located in your head — tells you the opposite.

It could happen anytime. While taking a jaunt down the street to the store, you run into someone, and bam, there it is. Or you’re in the middle of some sort of task at work — something you’ve done a thousand times before — but at this particular moment, it feels more familiar than it ever has.

French for “already seen,” déjà vu is more common in young people, and most individuals first experience it between the ages of six and ten. What separates it from just a normal feeling of familiarity is the fact that you know the familiarity is abnormal. The sensation is strong — too strong for it to make sense. It’s more uncanny and unsettling than it is warm and comforting.

And it has baffled scientists for centuries. The frequency of déjà vu experiences starts to decrease in most people after the age of 25 — when memory problems are actually in the uptick. But this trend could be interpreted as a sign that déjà vu isn’t a problem or disorder, and experiencing it is a normal part of a healthy brain.

With the great leaps in modern psychology research, scientists should now have an explanation for déjà vu, right? Well they might if they found an easier way to study it. There’s no magic wand or tool that will induce it in someone at will. It happens, and then it’s gone. If a researcher happens to be there to take notes and take an MRI or something, great. If not, better luck next time.

Still, there are a few ways to work around this obstacle, leading scientists to formulate a few theories about what causes déjà vu. One is anxiety. A case study of a 23-year-old British man afflicted with constant déjà vu suggests that his condition worsened when he felt more distressed about the experience.

Another idea espoused by Dr. Akira O’Connor, a psychologist at the University of St. Andrews, is tied to more biological reasons. O’Connor told the BBC in January that he believed déjà vu was caused by a “momentary ‘misfiring’ of neurons in the brain,” leading to connections that produced false memories. “Just as we get muscle spasms, or eye twitches,” he said, “it could be that the bit of your brain which sends signals to do with familiarity and memory is firing out of turn.”

Déjà vu seems to be more frequently experienced by epilepsy patients than the rest of the population. This provides a slightly easier path to studying déjà vu by way of neuronal discharges in epileptics. One study seems to pinpoint the rhinal cortex — which is thought to play a role in explicit memory, or the ability to recollect whether something is familiar — as the place in the brain where déjà vu originates, caused by aberrant electrical activity. Unfortunately, that study has never been replicated, and there’s no way to tell whether these results are specific for epileptics, or could be extrapolated to the rest of the population.

Across the Atlantic Ocean, psychologist Anne Cleary at Colorado State thinks of déjà vu with a cognitive lens. She suggests the experience is triggered by a particular object or sensation in the environment, and leads the brain to falsely believe it has experienced the same moment before.

Again, however, the hurdles in studying the phenomenon make it extremely difficult to actually test out this theory and others like it.

So for the most part, déjà vu is a mystery, and will likely stay that way until technology puts us in a place where scientists can study participants’ brains outside the lab. Perhaps the future of brain implant technology will allow people to not just observe brain activity when déjà vu happens, but to trigger it on the spot as well. This would be great for research, but it might also be the perfect way for a team of mad scientists to control the world by raining déjà vu down on us again, and again, and again, and again.