There were many ways to write a secret letter, such as using a mixture of aminophenazone tablets dissolved in water. Through a special process, handwriting written with that mixture could be displayed. Another example was to write with a mixture of starch water with rice water. After lightly wiping the paper with an iodine solution, the hidden message would also be revealed.
But Yang Xiaojin had chosen to use onion juice this time only because this method was relatively simpler. Besides, if someone wiped the piece of paper with an iodine solution first, they would not be able to see the handwriting clearly even if they heated it over an open flame afterwards.
When the onion juice was absorbed by the paper, it would start burning at a lower temperature than the paper. Therefore, the writing would turn brown when slightly heated.
