Not all blur is equal

I took my kids to the zoo at the weekend, one of their favourite  activities other than seeing the animals is playing on the Flying Fox. I have taken lots of images of them some with fast shutter speed to freeze the action but yesterday I was playing with a slower shutter and panning. I managed to get a few with them sharp and the background blurred but my favourite was this one, with nothing  sharp. 


1/8s @ f/20 ISO 800 24mm