"You can use a lemon to write a secret message.
You will need:
cotton swab (Q-tip)
What to do: Squeeze the lemon juice into the saucer. Add a few drops of water and mix well with a spoon. Dip the swab into the lemon juice. Then use the swab to write a message on ordinary white paper. When it dries, the writing will be invisible. When you want to read the message, heat the paper by holding it near a light bulb.
What happens: The words appear on the page.
Why: The juice of lemons and other fruits contain compounds of carbon. These compounds are nearly colorless when you dissolve them in water. But, when you heat them, the carbon compounds break down and produce carbon, which is black."
What happened when CKS tried it? My message didn't show up that much - it was clear and shiny and I could hardly see the letters. My mom used a lot of lime juice and it showed up well and it was a little yellow, but some other letters were clear. My advice when you try this is: Use a lot of lemon juice or lime juice to make this work.
CKS rating: 4 stars - Pretty fun!