Any mechanical contact will actually make and break many, many times on each closure. This is because the mass of the moving contact, combined with the spring inherent in all metals, will make it literally bounce off after the initial strike, and several times thereafter.
Jack G. Ganssle did some experiments and captured the waveforms of a bunch of switches, disproving many popular myths about switch bounce. In particular, elastomer button outputs don't "bounce" so much as very slowly rise and fall. The "bounce" on such elastomer button outputs can be eliminated with a Schmitt trigger between the button and the digital logic. "A Guide to Debouncing" http://www.ganssle.com/debouncing.pdf Jack G. Ganssle 2004, 2008
Debouncing Inputs with PICs
Debouncing Inputs with SXs
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