The audience learns that Mercutio is more of a realist than an idealist. Mercutio seems to be a bit fed up with Romeo's melancholy temperament over love and dreams; so, he mocks him by telling the legend of Queen Mab. Queen Mab is a tiny fairy who makes trouble for man and animals through dreams just to frustrate them or to make men run after dreams that are not true. Romeo, on the other hand, believes in omens, dreams, and premonitions, which Mercutio thinks are tricks of the brain (like Queen Mab is just a trick). Mercutio believes that he has control over his own life rather than being the mercy of dreams or of Fate. After Mercutio tells about Queen Mab, Romeo tells him to calm down. Mercutio responds with, "True, I talk of dreams;/Which are the children of an idle brain,/Begot of nothing but vain fantasy" (I.iv.103-105). Basically, Mercutio is saying that if anyone believes in dreams, it is only because their brain has nothing better to do than believe in fantasy, which never delivers the truth. He is the perfect foil to Romeo.