Invisible ink

If you ever want to write a secret message on paper, simply use vinegar, lemon, or onion juice, as the invisible ink. Write with it as usual on white writing paper. After it dries the writing is invisible. The person who receives the letter must know that the paper has to be held over a candle flame the writing turns brown and is clearly visible.
Vinegar, and lemon or onion juice, cause a chemical change in the paper to a sub stance similar to cellophane. Because its ignition temperature is lower than that of the paper, the parts written on singe.