Invisible ink has existed since at least the first World War, if not before. There are various types of invisible ink, and many different ways of making the ink visible. Some use chemicals to do this, some use UV lights, heat, etc.
Baby faeces shares some of invisible ink's properties. The baby's bottom and your hands can look poo-free, but a baby wipe brushed over the surface of the skin reveals the terrible, terrible truth...

Baby faeces shares some of invisible ink's properties. The baby's bottom and your hands can look poo-free, but a baby wipe brushed over the surface of the skin reveals the terrible, terrible truth...
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