We spend almost half of our waking hours doing it.
But we don't like to talk about it.
What is it?
The answer is daydreaming.
A 2010 study by psychologists at Harvard University in the United States that involved more than 2,000 volunteers revealed that we spend almost half of our time distracted by thoughts about things other than what we're actually doing.
Some of this is mundane stuff: Thinking about what to buy for lunch; mentally rehearsing a conversation we're about to have; or reflecting on something that has happened to us earlier in the day.
Some of it involves reminiscing about - or perhaps regretting - events from the past.
Some of it involves looking forward to - or perhaps worrying about - things that might happen in the future.
And some of it centres upon creative fantasies in which we re-invent ourselves and our lives - self-written, self-directed movies in which we take the role of a conquering hero, romantic hero or tragic hero.
Daydreams of this type are deeply personal.
A study conducted at the University of Minnesota in the US showed that 80 per cent of us would prefer to tell others about a genuine embarrassing incident that has happened to us, rather than reveal the contents of our daydreaming fantasies.
REGRETS ABOUT SHARING
I shared one of my daydreams once - and promptly wished that I hadn't.
I used to belong to a book club where members took turns to choose a book for everyone to read and discuss.
When it was my turn to choose, I plumped for the 1959 novel Billy Liar by Keith Waterhouse, an intensely sad and funny book about a young man, Billy Fisher, who compensates for his dreary, unfulfilled life by indulging in elaborate daydreams.
During our discussion, I confessed to being a chronic daydreamer, and even went so far as to reveal the contents of one of my most enduring and recurrent daydreams.
I foolishly expected that this would inspire other members in the group to open up and share their own daydreams.
Instead, it led to a long and embarrassed silence.
I made a mental note to, in future, keep my fantasy world to myself.
THE LIGHT AND DARK SIDES
Daydreaming can be a pleasant and rewarding activity.
It can help us understand ourselves, our desires and our hopes. It can aid us in plotting our futures and motivate us to pursue and achieve our goals.
It can keep us entertained and amused while we perform otherwise dull and routine tasks.
But daydreaming also has its dark side. Not all daydreams are positive and constructive.
Some of them - especially those that are negative, repetitive and focused on the past - can lead us to feel dissatisfied and unhappy with our lives.
According to the researchers who conducted the 2010 Harvard study, daydreaming - in the widest sense of the word - tends, on the whole, to have a negative impact on our emotions.
They found that the more time people spend distracted from what they are actually doing, and the more time they spend reminiscing, planning or daydreaming instead of engaging with the present, the less happy they are likely to be.
"Mind-wandering is an excellent predictor of people's happiness," said researcher Matthew Killingsworth, who led the study.
"In fact, how often our minds leave the present and where they tend to go is a better predictor of our happiness than the activities in which we are engaged."
It would reward us all, then, to cultivate the habit of living more often in the present.
Gary Hayden is a philosophy and science writer.
This article was first published on Aug 21, 2014.
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