( ( ( A bubble ) ( in water ) ) ( ( is ( ( a hole ) ( in ( a ( dielectric medium ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT A) (NN bubble)) (PP (IN in) (NP (NN water)))) (VP (VBZ is) (NP (NP (DT a) (NN hole)) (PP (IN in) (NP (DT a) (JJ dielectric) (NN medium))))) (. .)))
A bubble in water is a hole in a dielectric medium.
(ROOT (S (NP (NNP Gas) (NN bubble) (NNS holes)) (VP (VBZ helps) (NP (NP (JJ pumice) (NN float)) (PP (IN in) (NP (NN water))))) (. .)))