A postage stamp is a small, usually square piece of paper that letter senders employ to mail parcels. Prior to the 1840s, senders had to postmark their packages, and recipients often had to pay for delivery. All told, more than 200 nations and many ''micro-nations'' and ''imaginary nations'' produce postage stamps each year. Philatelists estimate that humanity has printed at least 600 hundred billion stamps over the last 165 years.

Philatelists, or people who study and collect postage stamps, employ numerous categorization methods. The British developed the concept of the ''self-adhesive stamp.'' Adhesive stamps can be wetted and then attached to letters. Creating these ''gummy'' stamps requires skill at both chemical engineering and printing. For the first 40 years of the adhesive postal stamp, workers had to apply gum to stamps by hand.

Finally, in 1880, a British company named De La Rue developed a manual process to attach adhesive to the backs of stamps. Most postal services attempt to spread the gum on the back of stamps as evenly as possible. That being said, in the 1940s, a local post in Germany intentionally left out patches of adhesive on some of their stamps.

Postal stamp adhesives run the gamut. Gums have been made from dextrin, gum arabic, certain kinds of gelatin, even a substance called polyvinyl alcohol, or PVA. A different post office in Germany tried to do away with the concept of paper postage stamps. This post printed a batch of experimental stamps made completely out of chemicals.