The Slinky Secret Codes kit is the fun way to learn about cryptology - the science of secret messages. Explore the methods behind codes, ciphers, hidden messages and more. Featuring duplicates of all contents, this complete cryptology kit for kids contains everything you need for you and a friend to send secret coded messages just like spies. There is even special ink markers for sending invisible letters. The Secret Codes kids kit includes 2 manuals, 2 cards, 8 wheels, 2 fasteners and 2 markers.