Every summer, 12-year-old Petra gathers lots of berries. She makes them into jam and tarts. In Sweden, all kinds of berries grow in the woods. You have to look carefully because some berries won't be ripe and taste very sour. Petra wants to know what makes the berries sour, and what sourness actually is. She does all kinds of experiments in a cabin in the woods to find out more about sourness. She uses sour berry juice to write a secret message, make plastic and produce loud bangs. But the best experiment can only be seen when it's dark: Petra makes tiny lights that light up the whole forest.

Originally released by Nederlandse Publieke Omroep, 2014. Streaming video file encoded with permission for digital streaming by Infobase on August 11, 2016.