A dad tracked down a knife-wielding thug who stole his son's iPhone by using an app to trace its location.
The thief, Aaron Clegg, snatched the teenager's phone after putting a knife to his chest and threatening him in a Liverpool park.
Clegg, 20, was tracked down when the father traced the phone back to Clegg's mother's address.
Chris Hopkins, prosecuting, told Liverpool Crown Court : "The two were in the park on August 9 this year. They were playing in the area of some exercise equipment and one had his phone out because he was using his phone to time his friend on an exercise machine.
"He was grabbed from behind by the defendant over his left shoulder – he could feel a knife held to him."
The court heard the two mugged teens both had mental health and learning difficulties.
The teenager described the moment as 'feeling as though the world had stopped'.
After taking the iPhone, Clegg then pointed his knife at a 17-year-old girl who was with the group and made her give him her Samsung mobile.
The court heard the first victim told his dad what had happened, resulting in him tracking the devices down to Clegg's mother's house.
Clegg's mum made him return the phones, but the police became involved when a row between them resulted in Clegg kicking her front door down.
Clegg pleaded guilty to two counts of robbery and a count of possessing a knife in a public place.
His legal representative said he was 'genuinely remorseful' and had been unaware his victims had mental health and learning difficulties.
Clegg was given a seven year extended sentence with four-and-a-half years in custody, of which he will serve at least two years and three months behind bars.