Chalk (media)
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In communications the term chalk refers to cylinders of the mineral crushed chalk, that has been processed to have physical properties consistent for it to be used to write temporary messages.
Chalk can be used to write on special chalkboards, or on many flat surfaces. Classrooms used to mount large chalkboards on their walls. Railway workers use chalk to write temporary messages on equipment, like boxcars. Chalk is a good choice for temporary messages as its small particles stick due to friction, and can easily be wiped away.