Shakespeare uses metaphors to project his thoughts upon dreams in Act 1, Scene 4. He does this by having Mercutio speak of dreams as the children of an idle/useless brain.
True, I talk of dreams,
Which are the children of an idle brain,
Begot of nothing but vain fantasy,we
Which is as thin of substance as the air
And more inconstant than the wind, who woos
Even now the frozen bosom of the north,
And, being angered, puffs away from thence,
Turning his face to the dew-dropping south.
In this scene Shakespeare uses Metaphors to persue his idea about dreams by saying that only ones that have nothing better to do;dream.He then sends out a completely different message with in Romeos statements about the dreams predicting his future.”Which are the children of an idle brain” is an example of how he has used a metaphor.
The children of an idle brain refers to him giving birth to children/thoughts as a result of having a small minded inactive brain.