Invisible Ink Secret Messages Experiment

Lemon juice (fresh or bottled)

Cotton swabs or a thin paintbrush

White paper

A warm lamp or iron (adult-supervised heat source)

A small bowl

Optional: white vinegar, milk, or baking soda solution for comparison

Squeeze lemon juice into a bowl — your invisible ink.

Dip a cotton swab in and write a secret message on white paper.

Let it dry completely (about 10 minutes). The writing disappears.

Reveal: hold paper near a warm lamp or iron on low heat. NO open flame.

Watch the writing appear in brown. "Heat caused a chemical reaction!"

Try other inks: vinegar, milk, baking soda water. Which works best?

Exchange secret messages with a friend or sibling.

A desk lamp with an incandescent bulb works well as a gentle heat source. LED bulbs don't produce enough heat.

Adult supervision is required for the iron or any heat source.

Write with plenty of lemon juice. Thin, dry strokes won't show up well.

Test different "inks" and rank them: which produces the darkest, most readable message?

This is a great introduction to pH and acids. "Lemon juice is acidic. What other foods are acidic?" (Vinegar, orange juice, tomato.)