Send invisible ink messages using lemon juice. In this clip from Australia’s The Curiosity Show, science educator and co-host Deane Hutton demonstrates how to write a secret message by painting with the lemon juice. He then reveals the dried invisible message by very carefully ironing the paper. “You’ve actually formed toffee in the paper.” More from KiwiCo:

“Lemon juice is mostly made of a variety of sugars, acids, and water. Normally, lemon juice is mostly clear, so when you dilute it even further in water, and paint that mixture onto some paper, you can’t see it. However, heating up lemon juice causes some of its sugars to react with oxygen in the air (oxidation), which turns these sugars brown. Acids in the lemon juice can also react with fibers in the paper to make more sugars, which again turn brown in the heat. Your invisible ink might seem like magic, but really, it’s all just chemistry.”

You can also use a candle or an incandescent light bulb. As always, be safe, smart, and consider adult supervision when conducting heat and fire experiments.

The Curiosity Show celebrated science, technology, and things to make and do from 1972 to 1990. Watch more short videos from the show on their YouTube Channel.

Then check out more experiment videos on TKSST.