A baby's first smile is an exciting moment. But what can it tell us about their understanding of the world?
Boasting about the speed of childhood development is the sport of choice for many a doting parent.
From the 12-week scan right through the early years, monitoring the physical and mental progress of their pride and joy is a source of both excitement and concern.
Especially rewarding is the onset of smiles, squeals and laughter - the kind of milestones that make all the disturbed nights worth it.
But is it all just wind?
Apparently not, as researchers now think that laughter and games like peek-a-boo could be telling us something more, and giving us a way to peer inside the workings of their minds.
"Laughter and smiles start incredibly early, just like tears," says Dr Caspar Addyman, a baby laughter researcher at Birkbeck College in London.
"So this leads us to think that it's a form of communication," he told the BBC.
Dr Addyman has collected nearly 700 questionnaires about baby smiles and laughter from around the world. He's still looking for more examples.
He found that babies are smiling in response to pleasant feelings much earlier than expected, which can be as young as one month old.
Soon after that, at between two to four months, social smiles develop that are used specifically to engage the parents.
He now hopes to take the research further and use laughter as a new way of tracking what it is that babies understand about the world around them.
The person who most greatly influenced our current view of childhood development was Swiss psychologist Jean Piaget.
Careful observations of children at different ages led him to identify four stages that everyone must go through to reach the cognitive abilities of an adult (see box).
In the earliest months of life, Piaget said that babies are only able to learn about the world by directly interacting with it through grasping, shaking and sucking.
With each experience, he concluded, children gradually build up a picture of how the world works - a kind of naïve physics.
But Dr Addyman thinks that studying babies' laughter can be just as effective at helping us pinpoint developments in the way their minds are expanding.
"You can't laugh at something until you get the joke, so what they laugh at really tells us about their understanding of the world," says Addyman.
As a result of having experienced so little, small children are for the most part quite content to accept the absurd as completely plausible.
Dr Addyman, who sports a crop of bright blue hair, sees this regularly with his young volunteers.
"Young babies would never laugh at my hair. But older children realise there's something wrong and that makes it funny."
The ability of children to suddenly see the funny side, Dr Addyman believes, acts to highlight much more profound developments going on deep inside their brains.
The Baby Laughter Project, which has surveyed parents from more than 20 countries, has shown that games like peek-a-boo are perfect for showing one such fundamental development - object permanence.
The term describes the understanding that an object still exists, even if you can't see it.
Very young children don't know this, which is why babies under around six months can look shocked and startled at peek-a-boo.
They think that not being able to see mum or dad's face means that they've actually disappeared, making their sudden reappearance come as quite a surprise.
However once a child understands (at around six- to eight-months old) that their parent is just hiding, then peek-a-boo becomes all about the anticipation of when they're going to come back.
Laughing as conversation
"Peek a boo is all the best things. It's mummy reappearing, but it's also about shared communication," says Dr Addyman.
"You can't help but smile and laugh when a baby starts laughing with you, which is really valuable for them developing their ability to interact with other people."
A potential link between laughter and language development suggests we've thus far underestimated babies' sense of humour.
Joking requires an element of taking turns as well as other skills needed for advanced interaction, like imitation and eye-contact.
"Children can pick up the rhythms of conversation through joking and playing games," says Dr Addyman.
In fact, smiles and laughter could be important communication tools for infants before they develop language.
Even primates seem to use laughter as a way of regulating social interaction.
"Laughter in chimps is predominantly used in play and it tends to be the youngest members of the group who have time for playing," says Dr Katie Slocombe, a primate cognition specialist at the University of York.
"It's been shown in chimps that laughter facilitates extended play, and we can't rule out that it has some sort of role in social bonding. When a chimp laughs, it seems to encourage their play partner to continue tickling or chasing them - just like a baby keeping an adult's attention for longer," she told the BBC.
So should we try harder to interpret the deeper meaning in those burbles and titters? Dr Addyman suggests a certain level of caution.
"Historically, we've certainly been guilty of projecting adult interpretations about what babies are laughing at, and it's a constant danger when doing this kind of research," he says.
"You really have to look at this scientifically and at a range of ages to really start to understand what's going on in their heads."