( ( An echo ) ( ( is ( ( sound waves ) ( ( bouncing back ) ( from ( a ( hard surface ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT An) (NN echo)) (VP (VBZ is) (NP (NP (JJ sound) (NNS waves)) (VP (VBG bouncing) (ADVP (RB back)) (PP (IN from) (NP (DT a) (JJ hard) (NN surface)))))) (. .)))
An echo is sound waves bouncing back from a hard surface.
(ROOT (S (NP (DT A) (-LRB- -LRB-) (NN n) (-RRB- -RRB-)) (VP (VBP echo) (SBAR (S (VP (VBZ is) (VP (VBN heard) (SBAR (WHADVP (WRB when)) (S (NP (JJ sound) (NNS waves)) (VP (VBP bounce) (ADVP (RB back)) (PP (IN from) (NP (DT a) (NN surface))) (SBAR (IN that) (S (NP (PRP they)) (VP (MD ca) (RB n't) (VP (VB pass) (PRT (RP through)))))))))))))) (. .)))