An engaging and colorful demonstration was developed that illustrates the utility of resonance theory in predicting the relative acidities of alcohols. The demonstration can be used as an introduction to exercises that provide students with practice in writing resonance structures and in predicting relative acidities. The demonstration exploits the pH-dependent structural and color changes that result in three naturally occurring dyes. Dilute basic solutions are used to write invisible messages and ethanolic extracts from henna, turmeric, and roses are used as developing solutions. The demonstration can be quickly, easily, and safely prepared for use and was found to be effective in attracting students' attention and leading to lively discussions of resonance stabilization, keto-enol tautomerization, and relative acidities. (Contains 2 figures.)