The teenage boy is grinding a skateboard on a steep rail.
An expert skater is shows off his skills.
Just because a teenage boy is grinding his skateboard on a steep rail does not mean he is an expert skater, nor does it mean that he shows off.
exists e1 s1 x1 x2 x3 x4.(n_boy(x1) & of(x1,x2) & n_teenage(x2) & Theme(e1,x3) & Actor(e1,x1) & v_grind(e1) & on(x3,x4) & n_rail(x4) & a_steep(s1) & Theme(s1,x4) & n_skateboard(x3))
exists p1 x1 x2 x3.(off(p1,x3) & n_skill(x3) & of(x3,x1) & n_male(x1) & exists x4.((x1 = x4) & n_show(x4)) & n_skater(x1) & of(x1,x2) & n_expert(x2))