Summer in New York. What joy looks like when you're watching.

My daughter laughed at something this August that I couldn't hear from where I was standing. She was across the park with two other children and whatever happened was visible from fifty metres away — the specific physical quality of a ten-year-old laughing hard, which involves the whole body, not just the face. I watched it happen and didn't know what it was about and it was one of the better moments of August.

There's research on laughter in children that I find genuinely interesting because it contradicts what you'd expect. You'd expect that children laugh more at things that are funny — jokes, pratfalls, absurdity. The research shows that children laugh most when they're in a state of social connection with someone they trust, and that the specific trigger for the laughter is often minor or even invisible to an observer. The laughter is generated by the connection, not by the funny thing. The funny thing is the excuse.

This is consistent with what Barbara Fredrickson's work on positive emotions calls 'broaden-and-build': positive emotions — especially shared positive emotions — expand the range of thoughts and behaviours available to the individual and build lasting personal resources. Shared laughter is the fastest available producer of this effect. Two children laughing together across a park are not just having fun. They are building something.