A prisoner in Strangeways had his jaw broken by another lag who made a bizarre comment about his 'dead nan'.
Samuel Jones, 26, punched the man twice 'very hard', bursting his nose and leaving him on a liquid diet.
The prisoner, a wing cleaner at the jail, had been hearing Jones make comments previously but had been ignored him.
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But Jones suddenly began attacking him, punching him to the face.
He hit back in self defence, but Jones got him in a headlock before prison staff intervened and restrained him.
The unprovoked attack followed a bizarre comment Jones made about the victim's 'dead nan'.
Manchester Crown Court heard the pair didn't know each other, and there was 'no evidence or suggestion' that his grandmother had passed away.
Jones had been suffering from auditory hallucinations, and believed that his victim shaking his cell hatch was a 'sinister act' and that he meant him harm, the court was told.
Jones, who was serving a 10 year sentence for wounding with intent, had another eight months added to his spell in jail.
Prosecutors told how two days prior to the attack, Jones had verbally abused the victim but he'd ignored it.
Then on June 27 last year, he heard Jones mumbling but again ignored him, later hearing a comment about his 'dead nan'.
Jones launched the attack and left his victim with a bloodied nose and a cut lip.
He broke the right side of his jaw, but didn't need surgery.
He was left in lots of pain and on a liquid diet following the attack, prosecutors said.
Defending, Michael Johnson said Jones was 'unwell' at the time and had been 'hearing voices'.
He was later transferred to a hospital and diagnosed with paranoid schizophrenia, the court heard.
Mr Johnson said there was elements of 'impulse' and 'spontaneity' to the attack.
Sentencing, the judge, Mr Recorder Rigby, told Jones: "You punched him very hard, and you punched him twice.
"You burst his nose, and you fractured his jaw.
"That was all done without any provocation.
"It amounts to a serious assault, causing serious harm.
"One cannot accept people assaulting prisoners or prison officers while they are in custody without there being a consequence."
Jones, of Stanley Street, Mold, north Wales, pleaded guilty to assault occasioning actual bodily harm.
