Why do particles gather in a bucket's centre?

Six-year-old listener Jonathan asks: "when you stir a bucket of water, I know the water is pushed to the outside; however, why do any particles end up the centre after the water has finished spinning? I have asked my Dad, but he doesn't know." Phil Sansom found someone who does know - and it's fluid dynamics expert Dan Nickstroem... Like this podcast? Please help us by supporting the Naked Scientists