Invisible InkScience brain teasers require understanding of the
physical or biological world and the laws that govern it. There
are two secret agents who want to send letters to one another
but think that someone might try to read them. The first agent
came up with an idea to send a letter to the other agent by
writing with invisible ink. He then put the letter in his
mailbox for delivery. When the other agent received the letter,
he opened it and began to read its contents. How was it possible
for the first agent to write with invisible ink and how was the
other agent able to read it? Answer1) The first agent wrote the
letter with the liquid of a squeezed lemon (citric acid). When
the paper is dried nothing can be noticed on the letter. 2) The
other agent held the letter over a flame and did not touch it.
The hidden ink (juice) becomes a reddish-brown color and the
words can be clearly seen. See another brain teaser just like
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