This document discusses gears and their uses. Gears are used to transmit motion or change the speed or direction of machines. They are easy to use in machines that work together and can be used in engines or mechanisms like bicycles or motors. Gears facilitate daily life and work in industries. Common machines that use gears include bikes, cars, clocks and trains. Gears come in external and internal types, and rack and pinion gears are also discussed.