But the paper has got another trick up its sleeve: watermarking. This is a pattern or picture that is embedded in the paper but can only be seen when light is transmitted through the paper—in other words, when you hold the banknote up to the light. Despite the name, this is not a water stain or an ink of any sort. It is engineered by creating small changes in the density of the cotton, so that different parts of the note look lighter and darker to produce a pattern—or, in the case of US bills greater than five dollars, the heads of presidents.
