Material:

• Vinegar or Lemon Juice
• Feather or thin paint brush
• Paper
• Candle
  Warning: Adult Supervision Advised

Instructions:

• Dip brush in vinegar or lemon juice and write note on paper
• Let Dry
• To Read the Message:
  • Pass paper back and forth, near the ball of the candle flame
  • the message will gradually appear
  • Warning: be careful not to get too close to the flame

Science

Why does the ink turn dark?

There is chemical reaction.

The ink chars at a lower temperature than the paper does.