I wasn't expecting to hear the f-word from my three-year-old in the back seat on a regular Tuesday.
While caught off-guard, I was probably more shocked we had made it that long without him parroting unfavourable language (despite us trying to temper it around him).
I was also curious as to whether parents swearing in front of their kids can actually cause harm.
Swearing at kids, versus around them
Benjamin Bergen, a linguist and cognitive scientist at the University of California San Diego, says it's all about context.
The author of What the F: What Swearing Reveals About our Language, Our Brains, and Ourselves, says there is an important distinction between swearing at children — which can be considered verbal abuse and can cause harm — and around them, which "doesn't seem to".
"The only profane words that demonstrably cause trouble are slurs," such as derogatory, racist, homophobic and sexist insults, Professor Bergen wrote in the Los Angeles Times.
"But there's no similar proof that exposure to ordinary profanity — four-letter words — causes any sort of direct harm: no increased aggression, stunted vocabulary, numbed emotions or anything else."
Eva R. Kimonis is a clinical psychologist and director of the Parent-Child Research Clinic at the University of New South Wales.
She says swearing at children can be a "form of emotional abuse" and linked to harmful outcomes for some children.
While our experts aren't aware of any research that indicates swearing around children is directly harmful, Professor Kimonis says it may be damaging in ways we don't expect.
"Even if a parent is OK with swearing at home, the problem is that children haven't necessarily learnt the who, what, when of swearing, and when it's appropriate.
"If parents are not teaching them when and how it's OK to swear, then they might be swearing at school as well," she says, adding that could cause them problems in the classroom.
Parenting expert and family counsellor Rachel Schofield says kids' swearing might also be harmful if they are using it "to be nasty".
"They might be using it in a way that is hurtful to others around them."
However, it is less likely kids will use curse words in that way, research shows.
Professor Bergen told ABC Radio National a long-term observational study that looked at children swearing in the "real world" found they were "overwhelmingly" using swearing in pro-social ways, such as making friends or to smooth social interactions.
Why children start swearing
Research has found that children's vocabulary of swear words expands rapidly from ages one to two, to ages three to four, in parallel to the general explosion in language in the toddler and preschooler years.
It found that by the time children began school at age five, they already knew 42 taboo words.
Professor Kimonis says children learn through imitation — watching what the adults around them do.
"When those adults model a behaviour like swearing, then the child is more likely to swear."
And our reaction to that will determine if they keep swearing or not, she says.
"If the reaction is positive, like where a parent laughs and doesn't give a consequence, then they're probably going to continue swearing."
What works for one family might not another
Professor Kimonis says whether it's OK for our kids to swear is not "a black [and] white issue".
"It's family and child specific."
For parents who don't want their children to use profanity, she suggests "planned ignoring".
"That's where we help a parent to ignore the behaviour by not giving it any attention.
"This only works if you pair it with giving attention when the child is doing an appropriate thing like talking respectfully."
There are experts who argue planned ignoring can be harmful, so parents should talk to a professional and decide what is most appropriate for their child.
Ms Schofield says for some families, it may come down to context.
"Is it just used as a way to express some frustration? Or is it used in anger?
"It's about the culture within the family; what's OK and what's not — every family gets to decide what is OK in their family."
