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Jean Zhou, of On the Spot Mosgiel Mini Mart, said a shop assistant in her mid-20s was behind the counter when the boy came into the shop and presented a knife while demanding money and cigarettes yesterday morning.
''He was a young boy. The girl yelled and he ran away.''
Sergeant Kevin Davidson, of Mosgiel, said the boy - confirmed as a 13-year-old - removed his hoodie to look at the security camera during the incident.
The boy later told the teacher about the incident and was taken by police for questioning, he said.
The matter would be referred to Youth Aid and other agencies.