How long has it been since you caught a student passing a note in class? Warning: in this activity you’ll be teaching your students how to write disappearing messages, so there’s no telling what kind of mischief will result.
This lesson plan will show you how to use thymolphthalein, ethanol, and sodium hydroxide solution to illustrate the effect of pH change on indicator color. The resulting solution can be used as disappearing ink (Ward’s Science disclaims all responsibility for aforementioned mischief). This is a fun one that takes about half an hour.