Jimmy Doherty wants to know why jelly won't set if chopped papaya is added to it, so he visits South Africa and discovers that the exotic fruit contains enzymes that can also help tenderise meat. 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 groundbreaking tech start-up in San Francisco that hopes to feed the world by growing fish flesh in a dish. Matt Tebbutt finds out why homemade ice cubes are cloudy, when the ones bought in shops are clear.