A creative agency from Croatia, Bruketa & Zinić, made a great use of thermo-reactive ink in an annual report for the food company, Podravka. The annual report houses a small recipe book nested within its pages. It was printed with invisible thermo-reactive ink, which appears when exposed to heat.
To read the book the recipient must wrap it in aluminum foil and bake it in the oven at 100°C for 25 minutes. Empty pages become filled with recipes. This is a great use of this special kind of ink, only a serious cook can reveal the recipes. Undercook it, you get nothing!!! Overcook it, you burn it !!!