Is it true that that watermelon is white on the inside but turns red when it gets exposed to air as it's cut?
No it is not true. The red colour of watermelon is due to the presence of a pigment called lycopene which give watermelon its colour. However the colour is observed only when the fruit is in final stages of ripening.When watermelon is not ripen , it is white in colour. This is because the colour is overpowered by the colour of chlorophyll pigment. When fruit has ripen the chlorophyll pigment starts shedding off and thus we see the reddish colour due to pigment lycopene. However there is no such reaction which convert white colour of watermelon instantly to give a red colour.