1, the starch contained in the paper will react with iodine to generate iodine starch molecules, which are dark blue and purple; Lemon juice is rich in vitamin C, which will be colorless when it reacts with iodine. So the part where you write or draw with lemon juice will not change color.

2. Because lemon juice contains an ingredient called cellophane, writing on paper can reduce the ignition point of paper. As long as it is baked with fire, the part written by lemon juice will be burnt first and yellow words will appear.

We call this phenomenon carbonization reaction.

This question was experimented a long time ago, and it was used for secret letters and periodicals.