The weather has been dreadful and the cat has eaten something it should not have. We are all fine otherwise. Write soon.
Move the lamp closer
Lemon juice is mostly water and sugar. Dry it on paper and it disappears. Warm it and the sugar browns before the paper does, so the writing turns brown while the page around it stays white.
That is all it is. No chemistry set, no spy shop. Milk works, so does onion juice, so does the juice from an apple.
An adult holds the lamp, and never a flame. A warm bulb, an iron on a low setting, or a radiator will do it. Push the slider too far and you will see what happens to the letter.
How to make someWhy bother, when a cipher is stronger?
Because a cipher announces itself. A page of letters in groups of four tells whoever finds it that you have a secret, and sometimes that is all they need to know: not what you said, but that you had something to say.
A letter about the cat tells them nothing. And the two together are stronger than either, which is why real agents did both: write the dull letter, hide the real one between the lines, and encipher that in case the paper is warmed by the wrong hands.
Invisible ink is thousands of years old. Pliny the Elder wrote about using the milk of a plant, and every generation since has rediscovered it with lemons.
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Longer, harder and written for grown-ups. Worth a look when this page runs out.