In my entry about the slime I mentioned fluorescein as dye. Today I am going to explain how I made it.
Most synthesis instructions recommend zincchloride as catalyst but some drops of sulfuric acid work as well. (Sulfuric acid and zincchloride are both hydrophile, which is the needed effect).
15g phtalic anhydride an 22g resorcinol are ground to a fine powder and placed in a test-tube and heated. I used an alcohol burner but i recommend a hot air gun.
When the powder melts and turns into a thick, red, honeylike substance stop heating and let it cool down.
The fluorescein can then be dissolved in a small amount of water resulting in a dark red/brown solution.
One-two drops of this solution are enough to colour 500ml of water to a bright green.
The best results are achieved when the coloured water is viewed in sunlight.
A more detailed synthesis manual can be found at LambdaSyn.