A 16-year-old boy has been found guilty of killing a shopkeeper who refused to sell him Rizla papers.
Vijaykumar Patel, 49, suffered a "catastrophic" head injury in an unprovoked attack outside the minimarket where he worked in Mill Hill, north London.
The teenager, from Brent, went into the shop with two friends and tried to buy cigarette papers, but they were sent away for being under 18.
Prosecutor Louis Mably QC had told a jury that the teenager became aggressive and shouted abuse before hurling himself at Mr Patel outside the shop.
The attack was captured on CCTV and "it is possible to see the moment Mr Patel was hit and which boy delivered the blow", he added.
"This boy suddenly lunged at Mr Patel, flew towards him and smashed him in the upper part of his body with his right forearm," he said.
"When the boy smashed into the lower part of his face it caused Mr Patel, with his hands in his pockets, to fall backwards on to the pavement and smash his head.
"It was that moment, that blow causing him to smash his head, that caused his death."
Mr Patel "had not really been involved" in the incident and "was standing with his hands in his cardigan pockets watching what was going on" when he was struck, Mr Mably added.
Another shop worker then chased the boys away with a billboard sign and a broom.
The boys were seen "laughing, joking and happy about what had taken place" as they fled the scene, Mr Mably told the jury.
Mr Patel was found by police unconscious and bleeding in the street on January 6.
He was rushed to St Mary's Hospital in central London and died the next day from an injury to the back of his head.
The defendant, who cannot be named for legal reasons, denied manslaughter, saying he acted in self defence.
Following a week-long Old Bailey trial, the jury found him guilty and sentencing was adjourned to September 7.
Detective Chief Inspector Luke Marks, from Scotland Yard, said: "It beggars belief that a family man with two kids has lost his life over an argument about cigarette papers that got out of hand.
"This was an unprovoked, spontaneous incident sparked entirely by the refusal to let the suspects buy what they wanted.
"A man has lost his life for no reason other than trying to uphold the law, which is there to protect everyone - including young people. "