( Calories ( ( are ( ( the measurement ) ( used ( to ( express ( ( the energy ) ( delivered ( by food ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNS Calories)) (VP (VBP are) (NP (NP (DT the) (NN measurement)) (VP (VBN used) (S (VP (TO to) (VP (VB express) (NP (NP (DT the) (NN energy)) (VP (VBN delivered) (PP (IN by) (NP (NN food))))))))))) (. .)))
Calories are the measurement used to express the energy delivered by food.
(ROOT (S (NP (DT A) (NN calorie)) (VP (VBZ is) (VP (VBN used) (S (VP (TO to) (VP (VB express) (NP (NP (DT the) (NN amount)) (PP (IN of) (NP (NN energy)))) (PP (IN in) (NP (NN food)))))))) (. .)))
A calorie is used to express the amount of energy in food.