Send one of these letters to a friend, family member, Valentine, or loved one and be sure to reveal the awesome science behind decoding them!
Materials: Paper, lemon juice, a cotton swab, heat source (light bulb or flame, with parent’s permission)
Squeeze lemon juice into a small bowl.
Dip the cotton swab into the lemon juice and use it as your pen. Begin to write your message on the paper. It appears invisible, right?
To decode the message, expose the paper to heat. Hold it under direct light or a flame without burning the paper (be sure to get your parent’s permission before handling fire) and voila! Your message has appeared.
How does it work? Lemon juice oxidizes, or combines with oxygen when heated. This makes it turn brown. If the message is still visible from the start, adding water to the lemon juice makes it harder to see before it is heated. Also, it works best to hold the message slightly above a candle.