 A candy called Toxic Waste holds the record for the world's sourest. They're so acidic that the package suggests brushing your teeth immediately after eating one so that it doesn't destroy all your enamel. Today is World Listening Day. Take a minute to stop and listen to all the sounds of nature. It's also National, or I should say it's Nelson Mandela International Day, commemorating the birthday of the civil rights icon. Nelson Mandela died in 2013.