A 14-year-old boy laughed and ran away after fatally stabbing 12-year-old Ava White, a court has been told.
A "chance encounter" became violent after an argument arose over a Snapchat video, with the defendant reportedly filming Ava and her friends without their permission.
More than 20 members of Ava's family and friends were in court for the start of the teenager's trial at Liverpool Crown Court on Monday, where the defendant - who cannot be named for legal reasons - appeared by videolink.
He denies both murder and an alternative count of manslaughter.
Charlotte Newell QC, prosecuting, said Ava and the defendant had been in Liverpool city centre with friends on the evening of 25 November last year and met "by chance encounter".
The defendant was in possession of a flick knife with a 7.5cm blade - and had pleaded guilty to having an offensive weapon.
Ava and her friends, all aged between 11 and 15, had been in the city centre watching the Christmas light switch on.
They had shared some alcohol and been "messing around" near the Royal Court Theatre before being told to move on by security staff at about 8.15pm.
They moved to Richmond Street where the defendant and his friends began filming them, later sharing the footage on Snapchat.
Ava and a friend approached the boys and asked them to stop filming and delete the footage, before being joined by more of her friends, including two older boys.
The jury was told the older boys left but the defendant and his friends then "jeered" at Ava, causing her to run towards them.
Family in tears as final footage played
Ms Newell said Ava had approached the defendant but did not "inflict any violence upon him greater than a push, nor did she produce or have any weapon".
Family members were in tears in the public gallery as footage was played to the court of Ava's final moments as she approached the teenager.
Unarmed Ava was then stabbed in the neck with the "force of a firm punch".
The defendant claimed he acted in self-defence, fearing he would be assaulted or stabbed.
But the prosecutor said: "His reaction at the time was to smirk, to laugh and to run away, leaving Ava to die whilst he sought to distance himself from his actions."
The court heard that he "began a conscious cover-up" of the crime, discarding his knife, phone and coat.
Mrs Justice Yip told the jury of eight men and four women that the defendant, who was accompanied by an intermediary, had attention-deficit/hyperactivity disorder and they may see him using a fidget toy to help his concentration.
The trial continues - and is expected to last three weeks.