During this same period, Godtfred Kirk Christiansen's son, , joined the managerial staff of the company, 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 in "LEGO family" sets, which went on to become the biggest sellers at the time; in the same 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. A LEGO production plant was opened in Enfield, Connecticut in the United States.
"Expert Series" sets were first introduced in , geared towards older, more experienced LEGO builders. This line soon developed into the "Expert Builder" sets, released in . These 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. Finally, the LEGO world came together in with the addition of the LEGO "". These small LEGO people have posable arms and legs, and a friendly smile. The figure was used in many varieties of LEGO sets, allowing consumers to construct elaborate towns with buildings, roads, vehicles, trains, and boats, at the same scale, and populated with the smiling minifigure LEGO citizens.