when the explosions start , they fall to pieces 
(ROOT (S (SBAR (WHADVP (WRB when)) (S (NP (DT the) (NNS explosions)) (VP (VBP start)))) (, ,) (NP (PRP they)) (VP (VBP fall) (PP (IN to) (NP (NNS pieces))))))
(ROOT (S (SBAR (WHADVP (WRB )) (S (NP (DT ) (NNS )) (VP (VBP )))) (, ) (NP (PRP )) (VP (VBP ) (PP (IN ) (NP (NNS ))))))