David Butcher, age 9, of Fargo, N.D., for his question:

WHEN WAS THE FIRST ESCALATOR USED?

An escalator is a moving stairway that you'll find in stores, air terminals, some office buildings and bus and train stations.

The world's first escalator was installed by the Otis Elevator Company at the Paris Exposition in 1900. In 1901, the same machine was reinstalled in Gimbel Brothers' department store in Philadelphia. It continued in use until 1939.

The steps of an escalator run on an endless belt. At top and bottom,

the steps fold so that two or three are level with the floor, making a moving platform that allows the passenger to step off without tripping.

Moving sidewalk escalators have also been built. Many are in use in airline terminals.