How to deal with a troll

In Scandinavian mythology, trolls were shady creatures who lived below ground and varied in size from giants (in Iceland) to dwarfs (in Sweden). They snatched infants and replaced them with baby trolls, or ‘changelings’, in an attempt to improve their breeding stock. They could, however, be tackled:
— By leaving a knife on a baby’s cradle, the trolls being frightened of iron.