You can use a lemon to write a secret message.
- The juice of half a lemon a cotton swab (or a toothpick wrapped in absorbent cotton or a dried-up pen)
What to do:
Dissolve the lemon juice in water and dip the swab into it. 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.
The words appear on the page in black.
The juices of lemons and other fruits contain compounds of carbon. The 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.
Be careful not to get the lemon juice or vinegar in your eyes, it will burn