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In the first episode of the hit “This American Life” podcast “Serial,” which focuses on the investigation of a 1999 murder in Baltimore, host Sarah Koenig says, “It’s really hard to account for your time, in a detailed way I mean. How’d you get to work last Wednesday, for instance? … Did you go to any stores that day? If so, what did you buy? Who did you talk to?” She emphasizes this because it’s actually pretty hard to remember what happened on a day like any other, whether it was last week or six weeks ago — as many of those involved in the investigation had to do. A new study shows why this happens; our brains just aren’t capable of remembering most things we do unless we consciously start trying, Medical Daily reports.

“It’s commonly believed that you’ll remember the specific details about the things you’re attending to, but our experiments show that this is not necessarily true,” said Brad Wyble, an assistant professor of psychology at Penn State University, in a press release. “We found that in some cases, people have trouble remembering even very simple pieces of information when they do not expect to have to remember them.”

The researchers call this concept of memory “attribute amnesia,” which means our memories are selective and don’t always remember pieces of information even when they were used moments ago. It’s the reason why our brains play tricks on us when we’re asked to remember certain events, and we end up fabricating certain aspects of it — typically the details in between the broader components of the story. As Koenig puts it, “If some significant event happened that day, you remember that, plus you remember the entire day much better. If nothing significant happened, then the answers get very general — ‘I most likely did this or I most likely did that.’” Though you don’t need a significant event to remember details, it’s the significant event that flipped that switch telling the brain to remember more details.

For the study, Wyble and his colleague Hui Chen conducted four variations of the same experiment on 100 undergrad students. Each group was asked to look at four characters arranged in a square, such as three numbers and one letter, and then asked to locate the corner the letter was in. After some time, the characters disappeared and the participants reported where the letter was located. They did this several times before being prompted to answer an unexpected question meant to probe their memory for the information they used to find the letter’s location.

Once they had answered the question, the computer then showed four letters on its screen, and asked the participants to mark the letter they had been locating for the entire experiment. Only 25 percent of participants were able to identify the letter — the same percentage researchers say would have been able to randomly guess. These tests were also conducted using odd numbers, even numbers, and colors.

“This result is surprising because traditional theories of attention assume that when a specific piece of information is attended, that information is also stored in memory and therefore participants should have done better on the surprise memory test,” Wyble said. This happened because the participants weren’t consciously trying to remember the details — another experiment on the same groups of people found that once the structure of the test was no longer a surprise, the average of correct answers jumped to between 65 and 95 percent.

“It seems like memory is sort of like a camcorder,” Wyble said. “If you don’t hit the ‘record’ button on the camcorder, it’s not going to ‘remember’ what the lens is pointed at. But if you do hit the ‘record’ button — in this case, you know what you’re going to be asked to remember — then the information is stored.”