SVT has previously reported on the campaign that spread like wildfire in the international media.

The campaign attracted attention in Japan, India, the United States, New Zealand and the United Kingdom, where it was somewhat jokingly described as "introverted Luleå residents" starting to learn how to start greeting each other.

The British newspaper The Guardian tested the experiment itself.

In the clip above, we get to the bottom of how the campaign could become so big.

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SVT's reporter tried cheering on people – see how the reactions were. Photo: Viktor Lundmark/SVT