"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 recieves 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 substance similar to cellophane. Because its ignition temperature is lower than that of the paper, the parts written on singe."
The first attempt was a fiery, smoky mess. Only milliseconds before the flame reached where I wrote "Skeptic" did it start to change color. I consulted the book. Was there something I missed? No, nothing. So I tried again, except this time not using the image as a guide. If I held the flame up to the paper in short intervals without letting it catch fire, it worked! Invisible ink! Observe.