( ( ( ( ( The answer ) ( is simple ) ) : ) ( it ( is ( ( the earth ) ( that ( is ( ( rotating ( on ( its axis ) ) ) ( once ( every day ) ) ) ) ) ) ) ) ) . )
(ROOT (S (S (NP (DT The) (NN answer)) (VP (VBZ is) (ADJP (JJ simple)))) (: :) (S (NP (PRP it)) (VP (VBZ is) (NP (NP (DT the) (NN earth)) (SBAR (WHNP (WDT that)) (S (VP (VBZ is) (VP (VBG rotating) (PP (IN on) (NP (PRP$ its) (NNS axis))) (NP (RB once) (DT every) (NN day))))))))) (. .)))
The answer is simple: it is the earth that is rotating on its axis once every day.
(ROOT (S (NP (NNP Earth)) (VP (VBZ rotates) (PP (PP (IN on) (NP (PRP$ its) (NNS axis)) (ADVP (RB once))) (CC times) (PP (IN in) (NP (CD one) (NN day))))) (. .)))