runs out of steam after a half hour . 
(ROOT (S (VP (VBZ runs) (PP (IN out) (PP (IN of) (NP (NN steam)))) (PP (IN after) (NP (DT a) (JJ half) (NN hour)))) (. .)))
(ROOT (S (VP (VBZ ) (PP (IN ) (PP (IN ) (NP (NN )))) (PP (IN ) (NP (DT ) (JJ ) (NN )))) (. )))