This is because the substance contained in lemon reacts chemically with paper under the action of fire, producing a substance similar to transparent film. The ignition point is very low. If baked on the fire, brown handwriting will appear on the zoom surface after the steganography.

Generally speaking, paper contains starch, and lemon juice is rich in vitamin C. The reaction between vitamin C and iodine will produce colorless phenomenon, so the part written with lemon juice is invisible. Lemon juice contains an ingredient called cellophane, which can reduce the ignition point of paper when writing on it. When baking with fire, the writing part will be burnt first, and the words will appear.

Extended data:

There is also a professional paper, that is, heat-sensitive paper, which realizes the way of displaying characters through the action of heat.

The manufacturing principle of thermal paper is to coat fine powder on ordinary paper base, and its composition is colorless dye phenol or other acidic substances, and read the paper into a film. Under the heating condition (above 70 degrees), the film melts and the powder is mixed to produce color reaction, thus achieving the effect of displaying characters.

The quality of thermal paper can be identified by baking. The back of the paper is heated with a lighter. If the color of the paper after heating is brown, it means that the heat-sensitive formula is unreasonable and the storage time may be short. If the black part of the paper has fine stripes or uneven color patches, the coating is uneven.

Baidu Encyclopedia-Steganography

Baidu encyclopedia-lemon juice