We use clocks to measure time．Inside every clock there is a special part that works at the same，regular speed．In electric clocks it is a regular electrical signal．This controls the speed at which the hands move，or how often the numbers on the display change．Some older clocks have a swinging pendulum to move the hands at a regular speed．
△Clocks come in all shapes and sizes． Many are battery-powered．Some watches are“digital”．This means the time is shown as numbers in a window．Other watches have a dial and hands．
△A pendulum is a weight on the end of a string or rod． Each swing of the pendulum takes exactly the same time．
△This old-fashioned grandfather clock has a long pendulum to control the speed of its hands．
△Inside this watch there is a spring that is wound up very tightly． As it unwinds，it moves tiny wheels called cogs，and these m ove the hands．The wheels，springs and cogs are so sm all that watch-menders have to use fine tweezers to repair or move them ．
△Long ago，people used sundials instead of clocks． The dial is marked in hours． The sun shines on the pointer，which casts a shadow．This shows you the time．
