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Easy⏱ time20 min$ cost2

Invisible Ink

Write a secret message in lemon juice that appears only when gently heated.

Start building ↓
fig — working model Result first

The build

Step 01

Write the message

Dip a swab in lemon juice and write on paper.

Step 02

Let it dry

Wait until the writing is invisible.

Step 03

Apply heat

Warm the paper carefully near a lamp.

Step 04

Reveal

Brown letters appear as the juice scorches first.

Working Principle

Lemon juice weakens the paper where it soaks in, so those areas brown (oxidise) at a lower temperature than the clean paper around them.

The science behind it

A closer look

Gentle heating drives an oxidation reaction; the acidic, sugary juice darkens before the surrounding paper does.

Take it further

Variables to test

  • 1 Compare milk, onion juice and vinegar as inks.
  • 2 Which reveals fastest at the same heat?
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