( ( Air exerts ) ( ( pressure ( because ( gravity ( holds ( ( air molecules ) ( in ( ( the ( earth 's ) ) atmosphere ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Air) (NNS exerts)) (VP (NN pressure) (SBAR (IN because) (S (NP (NN gravity)) (VP (VBZ holds) (NP (NP (NN air) (NNS molecules)) (PP (IN in) (NP (NP (DT the) (NN earth) (POS 's)) (NN atmosphere)))))))) (. .)))
Air exerts pressure because gravity holds air molecules in the earth's atmosphere.
(ROOT (S (PP (IN At) (NP (NP (DT the) (NN earth) (POS 's)) (NN surface))) (, ,) (NP (NP (DT the) (NN air) (NN pressure)) (VP (VBN exerted) (PP (IN on) (NP (PRP you))))) (VP (VBZ is) (NP (NP (DT a) (NN result)) (PP (IN of) (NP (NP (DT the) (NN weight)) (PP (IN of) (NP (NP (NN air)) (PP (IN above) (NP (PRP you))))))))) (. .)))