article_snippet.sent: The biology teacher told The Local: 'It’s a very rare phenomenon. '
article_snippet.sent: It’s difficult to say how many times It happens, but It has only been reported a very few times.'
article_snippet.sent: Teacher Karstein Erstad said Teacher Karstein Erstad found reports of the same strange phenomenon taking place in Sweden in the 1920s.
highlights_snippet.sent: Teacher Karstein Erstad says the 'very rare phenomenon' happened in Sweden in 1920s .