Invisible ink is the stuff of romantic legend. Spies and clandestine lovers alike have used it for centuries to convey hidden messages to their partners. Although invisible ink is still used today for these purposes, it has taken on many more practical and fascinating roles in recent years.
Hundreds of liquids, from fruit juices to specially formulated chemicals, can be used as invisible ink. Here are seven of the most interesting and accessible types.
(thanks Andy)
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