scratching your head in amazement over the fact 
(ROOT (S (VP (VBG scratching) (NP (PRP$ your) (NN head)) (PP (IN in) (NP (NN amazement))) (PP (IN over) (NP (DT the) (NN fact))))))
(ROOT (S (VP (VBG ) (NP (PRP$ ) (NN )) (PP (IN ) (NP (NN ))) (PP (IN ) (NP (DT ) (NN ))))))