INVISIBLE INK

With this experiment, you can communicate with your friends with secret messages. Nobody know what you have written if you do not know the method to decrypt it!
You'll need :
Clear vinegar or lemon juice.
Paper.
A candle.
A toothpick.
PROCEDURE
1 .- It takes a stick and wet the tip with vinegar or lemon
2 .- Write on a piece of paper.
3 .- Let it dry out and the message will become invisible.
4 .- To view, the approach paper to the flame of a candle and read the message.
5 .- The liquid to be exposed to heat and then oxidized, it becomes visible.