( ( An octahedron ) ( ( is ( ( the shape ) ( formed ( by ( ( ( ( gluing together ) ( equilateral triangles ) ) four ) ( to ( a vertex ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT An) (NN octahedron)) (VP (VBZ is) (NP (NP (DT the) (NN shape)) (VP (VBN formed) (PP (IN by) (S (VP (VBG gluing) (PRT (RP together)) (NP (JJ equilateral) (NNS triangles)) (ADVP (CD four)) (PP (TO to) (NP (DT a) (NN vertex))))))))) (. .)))
An octahedron is the shape formed by gluing together equilateral triangles four to a vertex.
(ROOT (S (NP (NP (DT The) (CD eight) (NNS sides)) (PP (IN of) (NP (DT an) (NN octahedra)))) (VP (VBP are) (PP (IN in) (NP (NP (DT the) (NN shape)) (PP (IN of) (NP (DT a) (NNS triangles)))))) (. .)))