Jimmy Doherty wants to know why jelly won't set if you add chopped papaya to it. In South Africa, Jimmy discovers that the exotic fruit contains enzymes that can also help tenderise meat. But could they also help the human gut? Jimmy quizzes Holland and Barrett, who sell papaya extract as an after-meal supplement. In Hawaii, Kate Quilton visits America's only tuna auction to find out why some fish have dangerous levels of mercury, while others don't. She also visits a ground-breaking tech start-up in San Francisco who hope to feed the world by growing fish flesh in a dish. And Matt Tebbutt wants to know why homemade ice cubes are always cloudy, when the ones that you buy in shops are always clear. They're all made with tap water, so what's the trick?