The American artist Alexander "Sandy" Calder (1898-1976) is best known for his kinetic sculptures known as mobiles and for his monumental static pieces known as stabiles. Many a museum, including the East Wing of the National Gallery of Art in Washington, D.C., has a Calder mobile hanging in an atrium. Many a prominent office building, such as the Kluczynski Federal Building in Chicago, has a Calder stabile standing in a public plaza out front.

Calder was the child of artists; his mother was a painter, and his father was a sculptor who worked on public commissions. As a result, the family relocated frequently. The Calders encouraged their son to be creative and made sure that, wherever they lived, young Sandy had a work area and basic tools at his disposal. He used wire and sheet metal to make jewelry for his sister and gifts of sculpture for his parents. But as he approached college age, Calder was encouraged to study, not art, but engineering.

He attended Stevens Institute of Technology in Hoboken, N.J., just across the Hudson River from Manhattan, where his father was working on bas-reliefs for the arch in Washington Square Park. At the time, the only course of study offered by Stevens was that leading to the degree of mechanical engineer, which Calder received in 1919. He worked for a while as an engineer, taking his work seriously enough to grow a mustache, so that he would "look more like a seasoned engineer," as he recalled in his autobiography.

However, like a lot of entry-level engineering jobs, most of Calder's were not as challenging or stimulating as he had hoped, and he began to take art courses. But there was one engineering job that he continued to pursue, not only because it gave him a creative outlet, but also because it paid well. This job was designing children's toys, which he did very successfully. Combining his creative imagination and his engineering background, Calder devised wooden pull toys whose motions mimicked those of the real things. By using such kinematic devices as eccentric wheels and cranked axles, he was able to make the toys come alive. He continued to invent toys even after he moved to Paris and became part of the avant garde art community there. His artistic endeavors included making and performing a model circus, which exploited his engineering knowledge of balance and motion.

The engineering curriculum in the early 20th century involved considerable time spent at the drawing board, where students were engaged in mechanical drawing, drafting, and descriptive geometry exercises. Such experience had honed Calder's already sharp spatial visualization skills. Among his early artistic accomplishments were wire portraits. Just as he had mastered a technique of using a single continuous line to draw on paper, now he used a single piece of wire to render a subject's head in space. It was said of Calder that he was never without a pair of pliers in his pocket, and he often had some wire in his hands.

He also began to experiment with motion as a compositional element, and this led to tabletop sculptures whose components were connected to motors by belts and pulleys. These were Calder's earliest "mobiles," a term coined by the French artist Marcel Duchamp to describe the unique pieces of art that Calder created. These no doubt owed much to the time he spent in engineering laboratories, which in his student days still distributed power via belts. Soon, however, Calder began to create delicately balanced kinetic sculptures that were powered only by the breeze from an open door, an air vent, or a passing art patron.