Roller skating is the traveling on surfaces with roller skates. It is a form of recreational activity as well as a sport, and can also be a form of transportation. In fact, as the United States readied for World War II, the government entertained the notion to add roller skates as essential equipment to move infantry around Europe to save gas. Skates generally come in three basic varieties: quad roller skates, inline skates or blades and tri-skates, though some have experimented with a single-wheeled "quintessence skate" or other variations on the basic skate design. In America, this hobby was most popular first between 1935 and the early 1960s and then in the 1970s, when polyurethane wheels were created and disco music oriented roller rinks were the rage and then again in the 1990s when in-line outdoor roller skating, thanks to the improvement made to inline roller skates in 1981 by Scott Olson, took hold.