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Invisible ink

Lemon juice, a cotton bud and a warm lamp, with the safety steps written plainly. An adult holds the heat.

about 15 minutes an adult holds the lamp makes a working machine

What you need

Half a lemon, or milk
A cotton bud or a thin paintbrush
Plain paper
A warm lamp, an iron on low, or a radiator
An adult

How to build it

Squeeze the lemonA few spoonfuls into a cup. No water, or it will be too weak to see.
Write with a cotton budDip, shake off the drips, and write. Go over each letter once only.
Let it dry completelyTen minutes flat on a table. It should vanish.
Write the dull letter on topIn ordinary pen, between the invisible lines. This is the part people forget.
Warm it, do not burn itAn adult holds the paper near a warm bulb or runs a low iron over it. The juice browns before the paper does.
Never a flame, never unattended, and an adult holds the heat. If the paper starts to smell or darken at the edges, stop.

The template

Print this at 100 per cent, not fit to page, or the sizes will be wrong. The ruler line below tells you whether it printed correctly.

10 cmwrite the dull letter on the linesand the real one in the gaps between them
The bar above should measure exactly 10 centimetres. If it does not, your printer has shrunk the page.

Now send something

Write the boring letter about the weather in pen, and MEET ME BY THE GATE in lemon between the lines. Post it to somebody in the same house.

Where it comes from

Rome, and every generation since
The trick nobody stopped using

Pliny the Elder wrote about writing with the milk of a plant and warming the page. Prisoners have used lemon juice, urine and milk; wartime agents used chemicals developed in laboratories. The principle never changed: something that dries clear and darkens with heat, on a letter that looks entirely ordinary.

Why it works

Lemon juice is mostly water and sugar. Dried, it vanishes into the paper; warmed, the sugar browns before the paper does. Nothing is hidden by cleverness, only by the fact that nobody thinks to warm a letter.

When it goes wrong

It usually does, the first time. Almost always one of these.

Nothing appearsToo much water, or not enough heat. Use juice straight from the lemon and warm it for longer, not hotter.
The paper browns all overToo hot, or too close. Move it further away. If it smells, stop.
The writing is visible before heatingToo much liquid on the bud. Shake it off before you write.

Where to go next

Try it on screen firstBack to Make