The word kinetic means relating to motion. Kinetic art is art that depends on motion for its effects. Since the early twentieth century artists have been incorporating movement into art. This has been partly to explore the possibilities of movement, partly to introduce the element of time, partly to reflect the importance of the machine and technology in the modern world, partly to explore the nature of vision. Movement has either been produced mechanically by motors or by exploiting the natural movement of air in a space. Works of this latter kind are called mobiles. A pioneer of Kinetic art was Naum Gabo with his motorised Standing Wave of 1919-20. Mobiles were pioneered by Alexander Calder from about 1930. Kinetic art became a major phenomenon of the late 1950s and the 1960s.