Little girl blows bubbles in a public square.
Little boy blows spit at people who walk by.
There can either be a girl or a boy. He/she can either be blowing bubbles or spit.
Girl and boy are contradictory genders. Spit and bubbles contradict one another, in that one (spit) creates the result of another (bubbles).
It is either a girl or a boy. Bubbles and spit are two very different things to blow.