The boy is skateboarding on the stairs.
The boy is sitting on the stairs.
Sitting on the stairs is not the same as skateboarding on the stairs.
exists e1 x1 x2.(n_stair(x2) & n_boy(x1) & on(e1,x2) & Actor(e1,x1) & v_skateboard(e1))
exists e1 x1 x2.(n_stair(x2) & n_boy(x1) & on(e1,x2) & Actor(e1,x1) & v_sit(e1))