( Puberty ( ( begins ( when ( ( estrogen levels ) ( are ( at ( a peak ) ) ) ) ) ) ; ) )
(ROOT (S (NP (NNP Puberty)) (VP (VBZ begins) (SBAR (WHADVP (WRB when)) (S (NP (NN estrogen) (NNS levels)) (VP (VBP are) (PP (IN at) (NP (DT a) (NN peak))))))) (: ;)))
Puberty begins when estrogen levels are at a peak;
(ROOT (S (NP (NNP Estrogen)) (VP (VBZ causes) (NP (NP (DT the) (NNS changes)) (PP (IN of) (NP (NN puberty))))) (. .)))