Watermarks are an integral part of some papers.  They are formed in the process of paper manufacture.  Watermarks consist of small designs, formed of wire or cut from metal and soldered to the surface of the mold or, sometimes, on the dandy roll.  The designs may be in the form of an eagle, eagle in circle, or letters spelling out "Mexico" or "Gobierno Mexicano".  These pieces of metal--known in the paper-making industry as "bits" - impress a design into the paper.  The design sometimes may be seen by holding the stamp to the light.  Some are more easily seen with a watermark detector.  Using a small black tray into which a stamp is placed faced down and soaked with watermark fluid, or "ronsonal"(aka lighter fluid) the image will appear in the form of dark lines against a lighter background.  The dark lines are the thinner areas of the paper called Watermarks.