Secret Messages

What You Need

• Lemon juice
• A writing implement (Q-tip)
• Paper 
• Source of heat

What You Do

1. Write a message on the paper using lemon juice as the ink
2. Let the secret message dry
3. Hold paper up to light bulb, sunlight, iron (no steam!) or other source of heat
4. Watch the secret message appear in brown

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The Science Behind It:
Lemon juice is acidic on the PH scale. When the acid is applied to the paper, it weakens the paper. The acid remains in the paper after thejuice has dried. When the paper is held near heat, the acid part (which is more weak) burns or turns brown before the rest of the paper.

Source: http://www.ehow.com/how_7154640_write-secret-messages-lemon-juice.html