Spies Like Us

The students will visit the American History for Kids website and read about The Culper Spy Ring. The Culper Spy Ring was a small group of men and women organized by Benjamin Tallmadge who gathered information about the British Army's plans and reported that information to George Washington's Army. This was an extremely dangerous job and the group reported their information using "secret messages". One of the most notable members of this group was Nathan Hale. The group used a codebook and wrote their messages in invisible ink, using a secret code cipher, or using a "masked" letter hiding a secret message within the letter. (A transcript of the codebook is attached here for students to use to create their "secret messages" or to observe as a primary source for this learning activity).

Tell the students that today they will make their own "invisible ink" and send "revolutionary warnings" to each other.

Ask the question, "Why would a spy want to use invisible ink?" (To keep the information secret to everyone except the person who is meant to know the information.)

Place students into groups with no more than three students in each group and make sure each group has a plain white piece of paper, a cup of white milk, and a Q-tip. Tell the students to dip one end of the Q-tip in the milk and then write a message on their paper (related to a revolutionary warning). Groups should be sure not to tell any other groups their "secret message". All groups should set their papers aside to dry.  Pass out the Spies Like Us Exit Tickets (one per student). Students should complete these while waiting for their messages to dry.

While the milk messages are drying, the teacher can also explain to the students the science behind the invisible message. Explain to the students that a sympathetic ink uses a chemical reaction. A person would write with one chemical and the writing would disappear. By applying a second chemical to the first causes a chemical reaction and the writing would be made visible again. There were several sympathetic ink formulas available during the time of the American Revolution.

Tell the students the "invisible messages" we made today are a little different because instead of using a second chemical, we are going to use heat to cause a chemical reaction to make our writing visible. This is how it works: The acid in the milk weakens the fibers of the paper. When heat is applied to the paper, the weakened fibers turn brown faster than the fibers that are not weakened and the message is visible.

If the milk messages are dry, have each group exchange their milk message with another group. Using a hairdryer, make the message appear. 

If time allows, students can share their messages with the class.