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INVISIBLE INK

Invisible ink allows messages to pass
undetected in plain sight, making every 
innocuous-looking letter or postcard a 
possible means of sending secret messages. 
A visible message is written in ordinary
ink, with the invisible message written 
between the lines, or in the unused 
portion of the page. The simplest 
invisible inks are organic compounds -- 
vinegar, lemon juice, or even urine -- that are invisible at 
room temperature but turn brown when heated. The example 
pictured was created with a lemon wedge and a toothpick and was 
"developed" with an ordinary incandescent light bulb. More 
sophisticated inks can be read only when a second chemical is 
applied to the invisible ink, and others are visible only under 
ultraviolet light. 
Source=http://www.cnn.com/SPECIALS/cold.war/experience/spies/spy.gadgets/espionage/invisible.ink.html