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Materials:	Phenolphthalein 1% solution in isopropanol Glass cleaner with ammonia.  The ammonia is important... it is what makes the experiment work. Cotton swabs Writing paper
Notes:	Phenolphthalein is probably hard to obtain outside of a chemical or laboratory supply house. In place of the glass cleaner, a spray bottle containing household ammonia diluted ten to one with water will work as well. Like all chemicals, phenolphthalein and ammonia must be treated with respect and kept away from mouth and eyes.  Adult supervision is required.

Procedure:

1. One end of a Q-tip® or other cotton swab is dipped into the 1% phenolphthalein solution.  The swab is held by the dry end.
2. A "secret" message is written on the paper with the wet end of the swab.  As the isopropanol evaporates, all traces of the message will vanish.
3. To develop the message, the paper is sprayed from a distance of at least one foot with a light mist of glass cleaner with ammonia.  Please ensure that the spray is always directed away from anyone's face.  

Phenolphthalein is an "indicator" that is colorless in its normal or acid state.  When exposed to a base, such as the ammonia in the glass cleaner, it turns deep pink, which is its basic state.  Since the ammonia can evaporate off the paper, the reaction is reversable, and the phenolphthalein reverts to its colorless state.

When the spray hits the paper, the invisible writing will immediately appear in vivid pink.  As the paper dries the writing will slowly fade.  It may be brought back by re-spraying the paper with the glass cleaner.

WARNING: Never spray Windex toward anyone’s eyes.