( ( Clicking ( on ( the ( two-headed arrow ) ) ) ) ( ( will ( reverse ( the ( foreground ( and ( background colors ) ) ) ) ) ) . ) )
(ROOT (S (S (VP (VBG Clicking) (PP (IN on) (NP (DT the) (JJ two-headed) (NN arrow))))) (VP (MD will) (VP (VB reverse) (NP (DT the) (NN foreground) (CC and) (NN background) (NNS colors)))) (. .)))
Clicking on the two-headed arrow will reverse the foreground and background colors.
(ROOT (S (NP (JJ Reversible) (NNS reactions)) (VP (VBP are) (VP (VBN indicated) (PP (IN by) (NP (NP (JJ opposite-headed) (NNS kinds)) (PP (IN of) (NP (NNS arrows))))))) (. .)))
Reversible reactions are indicated by opposite-headed kinds of arrows.