because !
clapper
It's called a 'clapper'
a clapper
A clapper.
It's called a movie clapper.
its called a clapper board
its called a clapper board
clapper board
The Chinese bridge is called Kangshaun great bridge
Is it the Brooklyn Bridge
Its a bridge. Its called the Golden Gate Bridge.
You are probably thinking of the clapper or clapboard, sometimes also called the slate, clacker, or clapper board. The contemporary version of this device, which came into use with the advent of talking movies, is the digital slate. See the link below for more information.