When the ink is exposed to heat, such as under an iron or over a candle flame or 100-watt light bulb, the acids in the liquid turn a different color and the message becomes visible. Once the message dries on the paper, it is invisible.
These inks can be made from many types of organic liquid. One very simple form of invisible ink is heat activated. One way to do this (and answer the challenge) is to use invisible ink. What do you think? How would you answer the challenge? It can be fun to think like a spy and figure out ways to send secret messages. Your challenge is to leave a written message on a piece of paper in a way that people won't see the message, even if they stare right at it. ATTENTION, SECRET WONDER AGENTS: Today's Wonder, should you choose to explore it, is one of great mystery…and science.
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