It’s not a rock or a grenade but a flying can of fermented fish

Police in Motala, Sweden, are investigating the fishy case of a 52-year-old man who was awoken on Thursday morning by an open tin of fermented herring, known as Surströmming, flying through his bedroom window. As anyone who has ever smelled the strange delicacy will know, this was breakfast in bed with a cruel and unusual twist. Investigators in the southern town are believed to have strong suspicions as to who may have been behind the pungent missile launch. Police are classifying the herring grenade attack as property damage.
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The video below shows some British people opening a can of Surströmming.
Perhaps understandably, it contains NSFW language.