Writing with lemonade and baking with fire can reveal the mystery of science: lemon juice contains vitamin B1, vitamin B2, vitamin C and other nutrients, which will be invisible if you write down. It contains an ingredient called cellophane. Writing it on paper can reduce the ignition point of the paper. As long as it is baked with fire, the part written with lemon juice will be burnt first, and the yellow words will appear.

You can also use white vinegar, strong brine, milk and water to write a secret letter, and strong brine to write a secret letter: dip a pen in some strong brine and write words. After the concentrated salt water dries, the water evaporates, leaving the salt dry on it, resulting in a rough layer. At this time, lightly paint it with a pencil, and the pencil will leave traces on the rough salt surface, and the words will appear.

Extended information:

Invisible material technology refers to the technology and method of making weapons and equipment with invisible materials

It is neither a material in nature nor from Harry Potter's magic school. The material invented by British and American researchers is used to refract light and other electromagnetic rays around objects, so that these light and rays give people the feeling of "invisibility".

Reference: Baidu Encyclopedia-Invisible Material Technology