( ( ( ( return atom ) : ) ( ( the ( smallest piece ) ) ( of ( ( a substance ) ( that ( ( ( ( can ( exist alone ) ) or ) ( can ( ( combine ( with ( other substances ) ) ) ( to ( form ( a ( chemical chain ) ) ) ) ) ) ) ... ) ) ) ) ) ) . )
(ROOT (NP (NP (NN return) (NN atom)) (: :) (NP (NP (DT the) (JJS smallest) (NN piece)) (PP (IN of) (NP (NP (DT a) (NN substance)) (SBAR (WHNP (WDT that)) (S (VP (VP (MD can) (VP (VB exist) (ADVP (RB alone)))) (CC or) (VP (MD can) (VP (VB combine) (PP (IN with) (NP (JJ other) (NNS substances))) (S (VP (TO to) (VP (VB form) (NP (DT a) (JJ chemical) (NN chain)))))))) (: ...)))))) (. .)))
return atom: the smallest piece of a substance that can exist alone or can combine with other substances to form a chemical chain .... .
(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))) (. .)))