A shop worker pinned a knifeman to the floor after his sister was robbed.
Adeel Ahmed, 18, was working in the Sudellside Minimarket, in Sudellside Street, Darwen, and had let his sister Samia, 16, take the till for two minutes when the robber struck.
He waved a steak knife and demanded she hand over the till's £70 contents.
When Adeel discovered what had happened, he and two neighbours chased the robber down the street.
Adeel and another man tackled the robber, who was waving the knife at them, near Sudell Road, and held him until police arrived.
Adeel, a student at Runshaw College, said: "I cannot remember much of what the guy said when I was holding him, except that he was muttering 'I will get you'."
"It was just instinct. My dad said I was stupid because I could have been hurt.
"It could have been so much worse. I thought about what could have happened for a while after, but I have forgotten about it now. I have other things to worry about."
Another neighbour, who pointed out the robber and did not want to be named, said: "The man was waving a knife around. They did a really good job to hang on to him until the police got there."
PS Ann Scott, of Darwen police, said: "Mr Ahmed has obviously displayed a great deal of courage.
"Although we don't advocate it, he chased the man and he is to be applauded for his bravery which led to the arrest."
Adeel's father, Purves, said: "I'm very proud of my son, though I would always warn him against doing that.
"He couldn't have done it without the neighbour's help. Not many people have that type of public spirit."
He said that Samia had been unable to talk about the ordeal and had struggled to sleep since the incident.
A man has been arrested and charged with armed robbery.
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