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Fact of the Week
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Invisible messages were written in urine
Fact of the week
13 November 2023
Invisible messages were written in urine
Long ago, Roman spies fooled their enemies by adding secret messages to official documents – you could only find them if you already knew they were there. The messages were written in urine as it is slightly acidic and this would alter the paper invisibly. When heated up, the altered sections of paper would turn brown and the message would appear.

Find out more in:
100 Things to Know About the Unknown

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