While modern spies rely on the latest technological advances, many of the principles of espionage have remained unchanged for hundreds of years. For example, the East German secret police used invisible ink to pass hidden messages during the Cold War, and the decades-old formula was deciphered just a few years ago. Likewise, the CIA only recently released formerly classified documents outlining the various types of invisible ink used during World War I. Among them is an ink made from one of the same ingredients as our invisible ink project. So don’t think you need a sophisticated GPS (global positioning system) surveillance transmitter to conduct your own spying; submarines still use periscopes today and so can you!

Milk Carton Periscope


What You Need:

Milk Carton Periscope Materials

  • Two half-gallon milk or juice cartons
  • Utility knife or box cutter
  • masking tape
  • pencil or pen
  • ruler

What You Do:

(Ask an adult to do the cutting!)

  4. Repeat step 3 with your other carton.

What Happened:



Featured Kit
Crime Scene Investigation Kit

Crime Scene Investigation Kit
Solve the ‘Mystery of the Disappearing Diamonds’ with this crime scene lab kit! Explore the forensic science field and perform seven detailed experiments to uncover which suspect stole the stones. Projects include dusting and fuming for fingerprints, fingerprint analysis, chromatography & ink analysis, blood type analysis (using simulated blood samples), and fiber analysis using a flame test. Ages 12 & up.

Invisible Inks

Invisible inks, also called sympathetic inks, can be made with many different substances. Sometimes they appear when you heat them up; other times another chemical reveals them. Get creative and see how many kinds of invisible ink you can find.

What You Need:

  • Ink type 1 –  lemon juice, grapefruit juice, vinegar, milk
  • Ink type 2 – saltwater, pencil
  • Ink type 3 – phenolphthalein, Windex
  • Cotton swabs or paintbrush

What You Do:

  1. Use a liquid like lemon juice or milk as ink. Write a message on a piece of white paper using a paintbrush or cotton swab, dipping in the ink frequently. Let the message dry. Turn on the toaster and carefully hold your paper over it. The heat will make your secret writing appear!
  2. Dissolve as much salt as you can in 1/4 cup of hot water. Write a message with the saltwater and let it dry. Now rub over the message with pencil lead and watch it appear!
  3. Some sympathetic inks work when you apply heat, but others work because of acid-base reactions. Use phenolphthalein solution to write a message on a piece of paper and let it dry. After it is dry, spray it with some Windex. Since Windex is a base, it will turn the phenolphthalein bright pink.

What other sympathetic inks can you find? Which kind shows up best? Which kind lasts longest?

More Spy Science Projects

Dusting for Fingerprints

Forensics Fingerprint Kit

Science Links

Learn more about light and optics.

Discover more periscope history.

More Spy Fun

Measure your spy savvy with these interactive spy games.

Visit the CIA kids’ page and try to crack the code.

See how spy tools have changed through the years.