To make invisible ink, you only need a few ingredients - and a lot of patience! Grab a lemon and watch how it's done. Full-Time Kid is available to stream on pbs.org and the free PBS App, available on ...

The CIA released previously classified World War I-era documents Tuesday, and most had to do with invisible ink. (However, in Peter Finn’s story about invisible ink, the weirdest method of passing ...

So you want to open sealed envelopes without getting caught? Here’s the secret, according to one of the six oldest classified documents in possession of the Central Intelligence Agency: “Mix 5 drams ...

A lead-based ink can be used to print invisible messages that become legible when decrypted with a chemical trigger. Conventional ‘invisible’ inks are not very secure, because they are easily read ...