Invisible ink used to be a major part of espionage. It may seem low-tech and quaint, but people still use it to communicate secretly. For centuries, people have written messages on paper that can only be read with special glasses or a light. But how do they work? In this Wonder of the Day, we’ll take a look at how invisible ink works, as well as some fun ways to use it.

The longevity of invisible pens varies widely depending on the quality and type of ink, as well as how it is stored. It can last anywhere from a few days to years. To reveal a hidden message, you must either expose the paper to heat (like placing it over an iron or over a candle flame) or apply a chemical liquid like lemon juice or vinegar to it. Many invisible inks also have telltale signs that can alert a censor to their presence, such as pen scratches or unusually smooth paper or a change in the reflectivity of the paper.

While invisible ink may not protect you if someone sneaks a peek at your text messages or TikTok conversations, it can help you avoid having to discuss anything too scandalous with friends. It can also be useful when you want to confide in someone, but don’t wish for anyone else to hear what you are saying. You can also use it to write secret notes to your friend or to give spoilers for a movie or show without giving away the surprise.

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