( What ( ( happens ( when ( ( gravity forces ) ( flex rock ) ) ) ) ? ) )
(ROOT (SBARQ (WHNP (WP What)) (SQ (VP (VBZ happens) (SBAR (WHADVP (WRB when)) (S (NP (NN gravity) (NNS forces)) (VP (VB flex) (NP (NN rock))))))) (. ?)))
What happens when gravity forces flex rock?
(ROOT (S (NP (NNP Gravity)) (VP (VBZ causes) (S (NP (NNS rocks)) (VP (TO to) (VP (VB roll) (ADVP (RB downhill)))))) (. .)))