( ( To ( understand rainbows ) ) ( you ( ( have ( to ( start ( by ( understanding ( what ( is ( ( happening ( inside ( a prism ) ) ) ( to ( let ( it ( ( separate ( white light ) ) ( into ( its colors ) ) ) ) ) ) ) ) ) ) ) ) ) ) . ) ) )
(ROOT (S (S (VP (TO To) (VP (VB understand) (NP (NNS rainbows))))) (NP (PRP you)) (VP (VBP have) (S (VP (TO to) (VP (VB start) (PP (IN by) (NP (NP (NN understanding)) (SBAR (WHNP (WP what)) (S (VP (VBZ is) (VP (VBG happening) (PP (IN inside) (NP (DT a) (NN prism))) (S (VP (TO to) (VP (VB let) (S (NP (PRP it)) (VP (VB separate) (NP (JJ white) (NN light)) (PP (IN into) (NP (PRP$ its) (NNS colors)))))))))))))))))) (. .)))
To understand rainbows you have to start by understanding what is happening inside a prism to let it separate white light into its colors.
(ROOT (S (NP (DT A) (NN prism)) (VP (VBZ helps) (SBAR (S (NP (NNS people)) (VP (VBP understand) (SBAR (WHNP (WP what)) (S (VP (VBZ makes) (PRT (RP up)) (NP (JJ white) (NN light))))))))) (. .)))
A prism helps people understand what makes up white light.