The jurors in the Ana Kriegel trial have been warned not to speculate, guess or make up theories in relation to the case.
The judge is addressing them for a final time before they are asked to consider whether two teenage boys murdered the schoolgirl.
Mr Justice Paul McDermott told the jurors they should consider the case against Boy A first before moving on to his co-accused Boy B.
Both boys deny murdering Ana Kriegel, but the prosecution's case in relation to their alleged roles is very different.
Boy A is accused of violently sexually assaulting and murdering her in a derelict farmhouse in Lucan just over a year ago - while Boy B is accused of assisting by luring her from her home in Leixlip knowing what was to happen.
The judge told the jurors mere presence at a crime scene is not a criminal offence, but it is different if someone is found to have done something preparatory - that's an act of participation, he said.
In relation to Boy B, he told them they had to consider his state of mind when he called for Ana that day and brought her to the abandoned house.
He told them if they had any reasonable doubt that he had prior knowledge as contended by the prosecution, they must acquit.
Reporting by Frank Greaney