Fluorescein is the main ingredient in the highlighter and it will glows magically under UV light when it is added into the water When you prepare the fluorescein solution, sodium hydroxide is needed to make the fluorescein dissolve in the water (as fluorescein is not soluble well in water) Note: please be carefully when deal with sodium hydroxide as it is highly corrosive Music: 0:00 Turn on 1:08 Ave Maria