Baby laughter: What it reveals about your baby's brain
19 October 2013 Last updated at 02:53 BST
A baby's first smile is an exciting moment for any parent. But are they really happy or is it all just wind?
Researchers now think that laughter and games like peek-a-boo could provide a way to peer inside the workings of a child's developing mind.
Dr Caspar Addyman, a baby laughter expert at Birkbeck College in London, has collected nearly 700 questionnaires about baby smiles and laughter from around the world. You can send him yours.
He found that babies are smiling in response to pleasant feelings much earlier than expected - and that laughter could help us track what they understand about the world around them.
Health Check's Claudia Hammond went to find out what's tickling them.