its limit to sustain a laugh 
(ROOT (NP (NP (PRP$ its) (NN limit)) (SBAR (S (VP (TO to) (VP (VB sustain) (NP (DT a) (NN laugh))))))))
(ROOT (NP (NP (PRP$ ) (NN )) (SBAR (S (VP (TO ) (VP (VB ) (NP (DT ) (NN ))))))))