Required: Potassium thiocyanate (KSCN), FeCl3, Ferric ammonium sulfate (Fe(NH4(SO4)2), Potassium ferrocyanide K4[Fe(CN)6], paint brush, poster paper, beaker

Variation 1: Secret ink

Prepare a solution of potassium ferrocyanide and soak a poster paper with the solution (or paint a poster paper with the solution). Dry with a fan.
Prepare a solution of ferric ammonium sulfate. Soak a brush with it and write on the poster paper.

Variation 2: Bloody finger

Soak a poster paper as above with potassium thiocyanate solution and allow to dry.
Prepare in a beaker a solution of FeCl3.
Put the poster paper on the blackboard. Give the spectators a story of horror alchemy, dip one finger into beaker and "write with your blood".
Wash your hands after demonstration!