where their heads were is anyone 's guess . 
(ROOT (S (SBAR (WHADVP (WRB where)) (S (NP (PRP$ their) (NNS heads)) (VP (VBD were)))) (VP (VBZ is) (NP (NP (NN anyone) (POS 's)) (NN guess))) (. .)))
(ROOT (S (SBAR (WHADVP (WRB )) (S (NP (PRP$ ) (NNS )) (VP (VBD )))) (VP (VBZ ) (NP (NP (NN ) (POS )) (NN ))) (. )))