Invisible ink is a great gift for kids. It will keep them entertained for hours. The pens are easy to use and have great longevity. The pens are available in bulk, so you can always have one on hand. Invisible ink is not only a great gift for children but also for adults.
Invisible ink is a very old technology. It dates back to the 1BCE, when Pliny recorded that milk from the tithymalus was used to make legible inscriptions. In the Renaissance, many politicians and lords used it to communicate in secret. With the beginning of war, invisible pens were further developed. German spies used simple citrus juice for writing messages that could be revealed only by steaming. The British censors quickly caught on to this and began steaming all letters in order to determine if they were written in invisible ink, and then reading the hidden messages.
There are two main types of invisible ink: organic fluids and sympathetic inks. Organic invisible inks can be made with common household items like lemon juice or vinegar, wine, onion juice, and even bodily liquids. These inks work because they change the fibers in the paper that they are written on, so they burn at a low temperature and turn brown quicker than the surrounding papers. They can be revealed by heating the paper on a hot iron, in the oven or holding it near a light bulb.
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