Stamp collecting is the collecting of postage stamps and related objects. The government printed Ryukyuan postage stamps and passports. Wombats have featured in Australian postage stamps and coins. See Postage stamps and postal history of Aden. PhotoStamps are sold as a sheet of 20 postage stamps. Her face is on postcards of Berlin and 1989 German postage stamps. John Muir was featured on two U.S. commemorative postage stamps. The robin has appeared on many Christmas postage stamps. The platypus has frequently appeared in Australian postage stamps and coins. His likeness also appears on many postage stamps. Roosevelt has also appeared on several U.S. Postage stamps. Germany has also issued three postage stamps honoring Gauss. Jackson has appeared on several postage stamps. Linnaeus has appeared on numerous Swedish postage stamps and banknotes. Harrison was memorialized on several postage stamps.