There are big, tumultuous moments in our lives when everything changes - a death of a loved one, a birth of a loved one, a new job, losing an old job.
And there are big moments which change our lives when we don't realise it until much later.
The path not taken led us somewhere else, for better or worse.
Or that we missed the big chance with the love of our lives to our eternal regret.
Or that, in a blinding moment, we realised who the love of our lives really was because of the way she or he behaved in a crisis!
Small moments with big consequences.
One of the most intriguing evenings at the Canberra Writers Festival is called "Light Bulb Moment" where prominent people talk about these quiet epiphanies.
The actor and author William McInnes once sheltered from a storm and witnessed an interaction between two well-heeled shoppers and a down-and-out; the writer Meg Keneally snapped out of her teenage self-indulgence after meeting people who had known true suffering; the comedian Hung Le changed his life plan after watching a miserable musician from afar.
McInnes has suffered one big life-changing moment. He's written about the death of his wife, Sarah, through cancer and how he continues to remember her at odd moments, like when he is swimming in the sea.
"It is the beauty of the reef that does it, of course," he says.
"How she laughed so much as she saw the colours, the fish, of sharing it with her children. It is only a few moments, but the feeling is acute. Sadness that she is not here, not for myself but for her, for my children.
It's pretty cool being human.William McInnes
"I think of my wife. Of how much she loved this world here. How she laughed so much as she saw the colours, the fish, of sharing it with her children."
But the Canberra session is also about smaller moments of everyday revelation - "light bulb moments".
For McInnes, one such moment was "pretty beautiful. It's pretty cool being human".
It happened when he found himself huddling with strangers under an awning, sheltering from a brutal Brisbane downpour.
Right in front of this drenched group, a down-and-out lady started humming a Puccini opera aria - "O mio babbino caro".
All opera buffs know the Puccini aria - the most erudite of them (some would say snobbish) know it's from Gianni Schicchi. Those less in the know, recognise it from the popular movie A Room With A View.
As this unusual performance in the downpour occurred, a snobbish couple in the group of strangers started mocking the lady along the lines of "how does someone down on her luck know a tune from 'Room with a View'?"
The poor woman pointed out that it was actually from an opera, though the two shoppers didn't seem to know that. She then rushed off, clearly hurt.
The couple were mortified by their own rudeness and arrogance, and the husband rushed after the impromptu singer and apologised. Hugs were exchanged.
"It was one of those random moments. She was someone who was bereft of all possessions," McInnes says.
He also has lighter moments of revelation: spotting a naked actor bent over in front of him during a production so "I could see what he had for breakfast".
The actor's male glory was on display "like something in the stock pavilion at the Royal Show."
It made Mr McInnes realise that there was no grand design to human creation. How could anything so ungainly, absurd and inelegant have been created?
"Evolution it's got to be," he says.
Sometimes he watches people applaud at his plays and he wonders at the primal reaction where human beings just bang their hands together like neanderthals.
It surprises him: "I would have farted the national anthem in Chinese," as he puts it.
"Who was the first cave person to do that?"
McInnes will be joined on the Canberra panel by another man who is funny but with a deep sensitivity - a wise jester.
On April 29, 1975, comedian Hung Le was a nine-year-old boy in Saigon as the North Vietnamese victors approached. He and his family boarded a creaking, leaking prawn trawler with the clothes they wore, a box of dried biscuits and some seasickness pills.
"We locked the dog in the house and the dog was just screaming and screaming. That's one moment," he says.
But the journey led to another moment that has stayed with him until this day. For the child, the journey to Australia started as an adventure because he was too young to know better.
But in the middle of the terrible voyage, "when we were on this barge in the middle of the ocean and there was no food and no water and all these old people got a polyester cup and peed in it". They were so tortured with thirst that they contemplated drinking urine.
People gave us so much money. They hadn't seen an Asian guy playing out of tune - just me.Hung Le
Only then did he realise that this was a matter of life and death and that life had great value. "I thought, 'God, this is where we've got to"."
As he grew up in Australia, another small moment changed his life.
He had taken up the violin because "that's how you fit into the white man's world - play their music!"
But he said he was always slightly out of tune. He couldn't hear it but everybody else could.
And then, in a classical music concert, he realised finally that the classical life was not for him.
One of the rank-and-file violinists seemed to lack any enthusiasm, let alone joy. He just sat after the performance looking like he didn't want to leave. His whole demeanour said: "This is miserable".
The sight of the unhappy professional violinist persuaded Hung Le that it wasn't the job for him. Music, he knew at that moment, should be fun.
This epiphany pushed him to perform on the street for laughs - and to use his slightly out of tune style for comic effect. Passers by started to laugh, laughter heightened by Hung Le's antics.
"I started making it funny and fun," he says. "I found it stressful until I made it funny. People gave us so much money. They hadn't seen an Asian guy playing out of tune."
The route to being a comic started with that observation of a miserable orchestral musician. He barely knew it at the time but now he thinks it was a turning point.
Meg Keneally's turning point from spoilt teenager to sensitive adult came through meeting some of the Holocaust survivors her father, the author Tom Keneally, was interviewing as he researched what would become his novel Schindler's Ark (Schindler's List in the international version and the film of the factory owner who saved Jews from the death camps).
She said it was actually a process - "a series of moments". When she was 14 and 15 years old, she was angry that she wasn't allowed to wear lipstick and other face paint. There were so many grave injustices the teenager (in common with others) thought she suffered at the hands of tyrannous parents.
"My mother wouldn't let me wear make-up," she says.
"But through the process of my father's researching, I I met a number of Holocaust survivors. I saw the tattoos on their wrists.
"When one lady visited our home, she pulled up her sleeve and showed me her six-digit Auschwitz number. I was struck how the numbers had been gouged into her arm to cause maximum pain.
"Hearing their stories woke me up to the fact that I was a whingeing, entitled teenager."
She draws a wider conclusion which stays with her: don't get too obsessed with the minor miseries of your own life when there are genuine tragedies beyond your immediate experience.
"It means that you can never feel too sorry for yourself. That, for me, was a really seminal moment."
She learnt something else from her dad, though it doesn't count as a lesson for life, rather a lesson for writing.
When she was pregnant nearly 20 years ago, she decided she was going to try to write a novel and tried again a year later, but both times she decided not to take them any further.
Hearing their stories woke me up to the fact that I was a whingeing, entitled teenager.Meg Keneally on meeting Holocaust survivors as her father researched "Schindler's List".
In November, 2014, her father told her he had written 30,000 words of a book, The Soldier's Curse, and hesaid, "I've had this in my back pocket for six years. I'm never going to get around to it, but I really want it to get written. How about you and I write it together?"
That started her off on a new career as a writer. She rewrote the first draft and worked on the rest with her father.
Her father's invitation led her to write. She learnt that it was hard work.
"It's just a matter of plugging away until you've got the words, and revising and revising.
"There's no divine spark. It's something that everyone has the potential to do."
She concedes that there may be a few people who are "insanely talented" but her father's advice to just keep at it showed her that doggedness was the key.
"Don't give yourself a day off," she says. The curse of the blank page has to be defied.
- Meg Keneally, William McInnes and Hung Le will share their wit and wisdom at Lightbulb Moment on Saturday, August 24 (7.30pm) at the Great Hall at ANU. John Birmingham, Sasha Marianna Salzmann, Nicola Moriarty and Mikey Robins will also be on the panel.