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If you struck a hammer on a thick metal plate and were able to view it in slow motion, you would see it go through several bounces before it settles down and come to rest on the plate. An electrical switch has a set of spring-loaded metallic contacts, and when operated, the contacts close in a manner much like the hammer. High-speed photography captures them repeatedly closing and opening until they settle and close. 

This results in false triggering or multiple triggering like a button pressed multiple times. Switch bouncing is not a major problem when dealing with power circuits, but it causes issues on logic or digital circuits. Hence, we use switch debouncing circuits to remove the bouncing from the circuit. The basic idea is to use a capacitor to filter out any quick changes in the switch signal.