Nostalgia has made a comeback.

With all the #tbts and #fbfs—also known as Throwback Thursdays and Flashback Fridays—as well as that NewsFeed-topping Year in Review feature on Facebook, social networks seem intent on making its users remember the past. Even food-ordering app Seamless got in on the action late last year, sending out emails that relayed the (grimace-inducing) data of how many times users ordered Chinese takeout, probably at too late of hours.

There’s also Timehop, described as “#tbt every day,” which pulls a user’s social-media posts, tweets, and pictures from the same day a year or multiple years before to create a handy virtual time capsule that’s ready to be shared with others. Timehop cofounder Jonathan Wegener says the app has 14 million users, about half of which check the app every single day.

That, he likes to say, is more than the number of people who read The New York Times each day.

“What we tapped into was just nostalgia, which is a fundamental human element,” Wegener says. “We give you a context for reliving the past, and a context for discussing and sharing the past.”

Wegener says that while social-media users have learned how much they enjoy tapping into their old memories, the apps and tech they use don’t know how to deal with the past. “The Facebooks, Instagrams, Foursquares of the world are all about real time. They're all about what's happening right now,” he says. Meanwhile, our personal digital histories on the social media platforms are only growing with each new post. Timehop, Wegener says, was a way of re-evaluating what to do with all that personal content. “What do we do in the post-real-time world, basically?”

Of course, this isn’t the first time people are practicing and embracing nostalgia—it is, in fact, a fundamental human practice. As long as there have been cameras, there have been ways to develop photographs and keep them; as long as people made journals, they kept accounts of their experiences to relive them. “We do often feel that we are traveling back in time and revisiting the original event,” Elizabeth Kensinger, a professor of psychology at Boston College, says about looking back at our memories.

Much of the way humans process and record experiences has to do with our body chemistry—our brains, even while experiencing an event, are already preparing to remember it. “There is much overlap between the brain processes that occur when we initially experience an event and when we later remember that event,” Kensinger explained via email. “When we initially experience an event, many parts of our brain are working together to allow us to process the visual input, language, tactile sensations, and other elements of the event. Cells are firing in visual cortex, auditory cortex, somatosensory cortex, etc. When we later think back to that event, a particular part of the brain—the medial temporal lobe—helps us to reconnect those different elements.” In other words, experiencing something and remembering that experience are, to the brain, remarkably similar.

The hippocampus, part of that medial temporal lobe, is especially responsible for piecing our memories together, according to Linda Henkel, a psychology professor at Fairfield University in Connecticut who specializes in memory errors and distortions. “It takes the separate pieces of our memories and kind of glues them together in a way,” she says. For example, when speaking with someone, “I have auditory information, visual information, all kinds of things—but my memory is a coherent whole. I don't just [think] ‘Oh here's the voice, and here's what you look like.’ I kind of glue those things together.”

Still, our memories are inherently incomplete—more so “than any of us would like to think,” Kensinger says. Try, for instance, to recall an entire conversation, word for word. “Memories often feel to us like a replay of past events, but really, memories are constructions of past events. That construction process can be faulty.”

“We can emphasize the fact that our memories are bad, but our memory systems are adaptive,” Henkel says. Our brains try to fill in the holes in our memories as much as they can. “We form schemas, we form stereotypes, we form generalities about things, and we use those to recreate our memories,” even if they turn out to be inaccurate.

Our brains also rely on retrieval cues, certain triggers that help us remember parts of our experiences. That might include things like our diaries, or photo albums, or social-media timelines. But, Henkel warns, even relying on social media as our own personal time capsules isn’t a safe bet for accurate memories. In fact, the practice of keeping up with social-media habits—tweeting our daily moments, Instagramming that cup of coffee—could hamper the way people store memories of an experience. In a study published last year in Psychological Science, Henkel took participants on a tour of the local university museum. They were asked to observe and remember certain objects, and at random, photograph some of them. Later, when Henkel asked them to recall the objects, she found that people remembered fewer visual details for the ones they had photographed.

Based on this experiment, Henkel argues that when we photograph something, we end up relying on the camera as an external memory source. “With cameras, what we seem to be doing is outsourcing our memories—we expect that the camera's got it. ‘The camera's got the picture,’” she says.

The problem is, we end brushing aside the whole moment as well. “What we're doing is kind of dismissing the experience: ‘I don't have to think about it any further, I don't have to elaborate on it,’” says Henkel. “Part of this outsourcing of memory, it's not a stupid thing to do. It frees us up to pay attention to the next thing that's happening. But if we live our lives constantly click click click click [with our cameras], going from one experience to the next, we're never stopping for the moments that we want to cherish and remember.”

Basically, she says, “the photos can help us reconstruct our memories, but they don't take the place of the experience.”

Still, it turns out that photographing even the most minutiae details of our lives—that pretty avocado toast at brunch, for example—might prove memorable after all. Ting Zhang, a doctoral student at Harvard Business School, has been studying the process of rediscovering past experiences in our minds. What she’s found through a number of different experiments is that people don’t expect to remember the “ordinary” moments in their lives—the commonplace conversations, or the regular workday—in comparison to “extraordinary” moments like birthdays or holidays.

“People underestimate rediscovery because they are overconfident in how much of the present moment they will be able to remember in the future,” Zhang wrote in an email. But, when presented with remnants of old memories, even of the most ordinary of moments, they found pleasure in remembering the experiences. “[My fellow researchers and I] were surprised by how interesting and meaningful these ordinary experiences were to people,” she says. Zhang believes our current social-media habits might have a lasting effect on how we value the experiences we might usually consider as commonplace. “As more of our lives are automatically recorded for us,” she says, “it's important to understand what aspects of our lives to document and how we can think about when to retrieve and rediscover what we have documented.”

More than a year into Timehop’s existence, Wegener says he’s noticed an interesting trend among some of its users. “Once in a while, we see people do kind of crazy things like leaving themselves messages for the future or saying hello to their future selves, or racing themselves and going back to the same place they were a year ago to kind of complete the loop or make it an annual tradition type of thing,” he says. “We see behavioral changes. People change their behavior in the now based on what they did in the past, which is kind of cool.”