Make Disappearing Ink

Once upon a time on a high school field trip, my class visited a magic shop, where we got some disappearing ink. We had great fun spritzing each other with the dark blue ink, which became invisble after a few seconds of exposure in the air. The teacher who was our chaperone wasn't quite as amused as we were when her silk blouse was 'stained' with the ink. The color disappeared, but a visible spot remained from the liquid on a dry-clean-only fabric. Therefore, these instructions for disappearing ink are presented to you with the advice that you don't spray the ink onto anything that can't survive washing. Otherwise, disappearing ink is easy to make and can be used to demonstrate acid-base indicators.