Long story short, I'm planning an in-home escape room and want to create a puzzle where the guests find a vial containing some sort of chemical that they can dump into a water fountain. My intention then is that chemical will react with another chemical that was placed on rocks within the water fountain prior. Then, the reaction causes a code to be spelled out on the rocks within the water fountain.
Obviously, the code on the rocks / in the fountain shouldn't be visible until the other chemical is introduced into the water fountain and the reaction occurs. It also goes without saying that the chemicals should be relatively non toxic or not a major issue if someone accidentally gets some on their hands. My one college chemistry course did not quite equip me for this task! Thanks for any help.