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The 16-year-old was stabbed 28 times in a Cheshire park in February.
Prosecutor Deanna Heer KC said jurors might find it difficult to fathom how two children could do what was alleged but urged them not to look for an explanation.
The pair, who were 15 at the time, deny murder and blame each other.
In her closing speech, Ms Heer said the jury may be tempted to look for an explanation as "they are children after all", but added the prosecution do not have to prove why a crime has been committed, only that it has been.
"Don't be tempted to try and work out why they did what they [allegedly] did if you're sure they did it," she told Manchester Crown Court.
Ms Heer said both encouraged the other in the attack, adding the evidence showed both defendants participated in Brianna's murder by encouraging the other, by contributing to a plan to kill her, by taking part in the plan and by trying to conceal evidence afterwards.
Ms Heer said the prosecution accepted some of the messages the boy and girl, identified only as girl X and boy Y, exchanged were initially fantasies about killing people.
