What do you find? Write on a piece of paper with a brush or cotton swab dipped in flour solution. The flour solution is dry and the paper is blank.

The question is incomplete. Do you want to say why there are no traces on the paper?

The principle is simple. There is free starch in it. Starch is colorless, odorless and soluble in water. If you write with a brush dipped in it, it will naturally leave no trace after drying. However, starch turns blue when it meets iodine. During the Anti-Japanese War, it seems that someone wrote letters with millet porridge, and the reason is the same.