Lego system as children outgrow their Duplo bricks. The name Duplo comes from the Latin word duplus, which translates literally as double, meaning that a Duplo brick is exactly twice the dimension of a Lego building brick (2× height by 2× width by 2× depth = 8× the volume of a brick). Expansion (1970–1991) During the last three decades of the 20th century, Lego expanded into new areas of toy making and marketing. In 1971, Lego began to target girls by introducing furniture pieces and dollhouses. In 1972, Lego added boat and ship sets, with floating hull pieces and Godtfred Kirk Christiansen's son, Kjeld Kirk Kristiansen, joined the managerial staff, after earning business degrees in Switzerland and Denmark. One of Kjeld's first achievements with the company was the foundation of manufacturing facilities, as well as a research and development department that would be responsible for keeping the company's manufacturing methods up to date. Human figures with posable arms made an appearance in 1974 in "Lego family" sets, which went on to become the biggest sellers at the time; the next year, an early version of the "minifigure" miniature Lego person was introduced, but it was not posable and had no face printed on its head. The company opened its first North American Lego production facility in Enfield, Connecticut, in the United States that same year. In 1975, "Expert Series" sets were introduced, geared towards older, more experienced Lego builders followed by the "Expert Builder" sets in 1977. The technical sets featured moving parts such as gears, differentials, cogs, levers, axles, and universal joints and permitted the construction of realistic models such as automobiles, with functional rack and pinion steering and lifelike engine movements. In 1978 the Lego "minifigure" was added. The small Lego people had posable arms and legs, and initially a single head with a smiling face. The figure was used in many varieties of Lego sets, allowing construction of towns populated with the smiling minifigure Lego citizens. Lego also expanded into space with the creation of Lego Space sets with astronaut minifigures, rockets, lunar rovers, and spaceships, and into the medieval territory with the Castle theme. In 1979, Lego introduced the Scala series, featuring jewelry elements marketed towards young girls. Kjeld Kirk Kristiansen also became the president of Lego that year. Since the 1960s, educators had seen Lego bricks' productive potential as being an invaluable asset in helping