( ( Explain ( what ( ( sigma ( and ( pi bonds ) ) ) are ) ) ) . )
(ROOT (S (VP (VB Explain) (SBAR (WHNP (WP what)) (S (NP (NN sigma) (CC and) (NN pi) (NNS bonds)) (VP (VBP are))))) (. .)))
Explain what sigma and pi bonds are.
(ROOT (S (NP (DT A) (JJ double) (NN bond)) (VP (VBZ consists) (PP (IN of) (NP (NP (CD one) (NN sigma) (NN bond)) (CC and) (NP (CD one) (NN pi) (NN bond))))) (. .)))