Secrets of confidential ink, stealth ink
After writing and drawing, I saw nothing. Reason? Use security ink, also called invisible ink.Secret of the 'invisibility' letter of spies
The US Central Intelligence Agency (CIA) recently released classified documents showing that World War I spies had printed messages on their toenails and used lemonade to writeMaterials known as 'self-destructive timers' can cause ink to disappear
The secret behind these self-destructive molecules is that they need a small amount of energy to survive - and when that energy is gone, they will disappear.