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Time flies if you’re one of the following: old, busy, or having fun. Here are two reasons why: 

High concentration. This is the biggest reason for the contracted perception of time. When we’re acutely focused on something—an assignment, a full day’s work, recreation—we simply forget about the ticking clock, says Dr. Anthony Chaston of the University of Alberta. “The more attention the task requires, the faster time flies.”

Conversely, time slows to a crawl when we’re bored, uncomfortable, or detached from what’s going on. “The passage of time seems to vary depending on whether the subject is in an unpleasant or pleasant context,” a recent study found. “It drags when being criticized by the boss but flies by when conversing with our friends.”

Under the influence of emotions, the study concluded, humans can be extremely inaccurate in their judgments of time. But there’s another factor that speeds our perception of time—one that even compounds the effect.

Our age. Since adults have observed more events in life than children, not to mention more years, the time passes faster for former. “A child’s whole frame of reference is built on a couple years,” explains Dr. Bob Kane, who researches aging at the University of Minnesota. “Every increment is a bigger piece of their life. As they get older, every increment is a smaller piece as the denominator gets bigger.”

Furthermore, since adults work more, they don’t benefit from the typical mile markers that frequently help young people track time, such as spring break, summer vacation, and other watershed moments.

The result: grown-ups feel a greater loss of time than children. “You’re constantly asking yourself, ‘How many more things can I accomplish before my time is up?'” Kane says. “Each moment becomes a little more precious.”

So the next time you wonder where the time went, remember: your focus and age stole it. In the meantime, treasure it. Seek out uncommon activities that create new memories and lengthen the perception of time. And don’t make the same mistakes these people did.

This story first appeared on Dec. 30 2009