What you’ll need

  • two or three cotton swabs
  • paper (try different types, they don’t all work equally well)
  • 1 – 1.5 teaspoons of baking soda (= sodium hydrogen carbonate; usually found among baking products in stores)
  • 100 ml hot water
  • measuring cup
  • iron (use only with the help of an adult)
  • teaspoon

How to do it

1. Mix the baking soda and the hot water to make your secret ink.

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2. Dip the cotton swab into the ink and write down your secret message. Now you need to leave the paper to dry.

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3. Ask your parents to iron the reverse side of the paper at the highest temperature. As they do this, keep checking the other side with the secret message. As soon as the words start to appear, you can put the iron aside.

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4. Very soon (a few minutes maximum) your secret message will be visible!

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What's the secret?

Soda solution is slightly alkaline. When heated, sodium hydrogen carbonate becomes more alkaline and attacks the cellulose in the paper. So where you wrote using the baking soda “ink,” the paper is more sensitive and turns brown more easily in the heat.