( ( A Caustic ) ( ( is ( ( a substance ) ( which ( ( burns or ) ( ( eats away ) ( by ( chemical action ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NNP Caustic)) (VP (VBZ is) (NP (NP (DT a) (NN substance)) (SBAR (WHNP (WDT which)) (S (VP (VP (VBZ burns)) (CC or) (VP (VBZ eats) (PRT (RP away)) (PP (IN by) (NP (NN chemical) (NN action))))))))) (. .)))
A Caustic is a substance which burns or eats away by chemical action.
(ROOT (S (NP (NP (DT The) (NN action)) (PP (IN of) (S (VP (VBG burning) (NP (NP (DT a) (NN piece)) (PP (IN of) (NP (NN wood)))))))) (VP (VBZ forms) (NP (DT a) (JJ different) (NN chemical) (NN substance))) (. .)))