a dirty boy runs through the dirt.
A child runs.
child is a more general term - if he is a boy, he is also a child.
exists e1 x1 x2 x3.(n_dirt(x3) & through(e1,x3) & Actor(e1,x1) & v_run(e1) & n_boy(x1) & of(x1,x2) & n_dirty(x2))
exists e1 x1.(Actor(e1,x1) & v_run(e1) & n_child(x1))