The dry lemon juice is the invisible ink and it will react after contact with the light bulb to make dark letters appear.
Ask the students for a hypothesis as to what is causing the reaction. The students will reply either “light” or “heat” from the bulb. The instructor will experiment using the hot gun and the flash light and show it is the heat from the bulb that is important. The reaction is a combustion reaction that will leave behind a small amount of ash.
Ask for examples of combustion, and explain the importance of combustion reactions in our everyday lives.