Invisible ink was created by American revolutionaries on November 29, 1775.
General George Washington, commander of the American forces fighting the British Empire, was eager to find a way for secret messages from spies to be kept hidden.
James Jay, a doctor, provided the answer, by creating an invisible ink system that used two chemicals.
The first chemical was used to write the message, and the second one, after it was applied to the first, revealed the words.
Washington instructed his spies to write their invisible messages in the blank columns of popular books to make them appear less suspicious.