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?