There are two secret agents who want to send letters to each other 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?
Answer
1) 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 colour and the words can be clearly seen.
