( ( ( A molecule ) ( of water ) ) ( always ( ( contains ( ( ( ( one atom ) ( of oxygen ) ) and ) ( ( two atoms ) ( of hydrogen ) ) ) ) . ) ) )
(ROOT (S (NP (NP (DT A) (NN molecule)) (PP (IN of) (NP (NN water)))) (ADVP (RB always)) (VP (VBZ contains) (NP (NP (NP (CD one) (NN atom)) (PP (IN of) (NP (NN oxygen)))) (CC and) (NP (NP (CD two) (NNS atoms)) (PP (IN of) (NP (NN hydrogen)))))) (. .)))
A molecule of water always contains one atom of oxygen and two atoms of hydrogen.
(ROOT (S (NP (NP (CD 2) (NN hydrogen)) (, ,) (NP (CD 1) (NN oxygen) (NNS atoms))) (VP (VBP combine) (S (VP (TO to) (VP (VB make) (PRT (RP up)) (NP (NP (DT a) (NN molecule)) (PP (IN of) (NP (NN water)))))))) (. .)))
2 hydrogen, 1 oxygen atoms combine to make up a molecule of water.