An invisible lemon juice message, scrawled onto a piece of paper, can be developed by exposure to any heat source such as a radiator, an iron or a 100W light bulb.

Mix the water and lemon juice with the spoon. Dip the cotton bud into the mixture and write a message onto the white paper. Wait for the juice to dry so it becomes completely invisible. When you are ...

“This invisible ink can be used to record and protect confidential information by printing process,” Li says. He says that it could also be useful in devices that combine light with electronics.