's the butt of its own joke 
(ROOT (S (VP (VBZ 's) (NP (NP (DT the) (NN butt)) (PP (IN of) (NP (PRP$ its) (JJ own) (NN joke)))))))
(ROOT (S (VP (VBZ ) (NP (NP (DT ) (NN )) (PP (IN ) (NP (PRP$ ) (JJ ) (NN )))))))