Effect:
A blank sheet of paper is sprayed with a colorless solution and a message in red and blue appears.

What You Need:
- large sheets of paper (for really big posters, use table cloth paper that comes in a roll.)
- a spray bottle (e.g. used Windex)
- ferric chloride
- ammonium thiocyanate
- potassium ferrocyanide
- water

What You Do Before the Show:
- Prepare 0.1 M aqueous ferric chloride and place in the spray bottle
- Prepare 0.1 M aqueous ammonium thiocyanate (for red)
- Prepare 0.1 M aqueous potassium ferrocyanide (for blue)
- Paint the paper(s) with an appropriate message and/or picture(s) with the second or third solutions.
- Let the paper dry.

What You Do During the Show:
- Spray the paper with the ferric chloride solution.
- The message will appear in red and/or blue.

What is Happening:
- Ferric chloride reacts with ammonium thiocyanate to produce a red pigment.
- Ferric chloride reacts with potassium ferrocyanide to produce a blue pigment.

Hazards:
None to speak of other than ferric chloride solution dripping off of the poster paper.

Patter:
You can use this to say "Welcome", "The End", or any other message you'd like.