The conversation he had overheard had stimulated his curiosity.
( ( ( The conversation ) ( he ( had overheard ) ) ) ( ( had ( stimulated ( his curiosity ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN conversation)) (SBAR (S (NP (PRP he)) (VP (VBD had) (VP (VBN overheard)))))) (VP (VBD had) (VP (VBN stimulated) (NP (PRP$ his) (NN curiosity)))) (. .)))
The converstaion made him curious.
( ( The converstaion ) ( ( made ( him curious ) ) . ) )
(ROOT (S (NP (DT The) (NN converstaion)) (VP (VBD made) (SBAR (S (NP (PRP him)) (VP (VBZ curious))))) (. .)))