World Records are being set at a much faster rate in swimming than in other sports. At the Rio Olympics, British swimmer Adam Peaty managed to break the men's 100m breaststroke world record twice in two days. Tim Harford speaks to swimming coach, Rick Madge, about the reasons swimmers keep getting better results in the pool. Also, science writer Christie Aschwanden makes the case for the virtues of the 5,000 metre race. She says that in recent times it has become very popular for people to train to run a marathon. But when you look at the numbers, is the 5K a better distance? Presenter: Tim Harford Producer: Charlotte McDonald