The heat from the flame causes a chemical change in the dried ink.
The portion of the paper which absorbed the vinegar or lemon juice
chars at a lower temperature than the untreated paper, so the writing
shows up as a faint brown scorching.
1) If you're using a toothpick for a pen, write with the round end so you don't scratch the paper. 2) Don't press too hard! If you do, you'll make an impression on the paper that can be read even without decoding it.
Excerpt from The Jumbo Book of Science
by the Ontario Science Centre, used by permission of Kids
Can Press Ltd., Toronto.
