Button does not work?

The button doesn't work as expected. I used the digitalRead() function to read the state of the button, but the button state is not changed as expected when press and release button. I press the button once, but Arduino code detects several presses. Why?
ANSWER

Beginners usually get trouble when getting started with a button, pushbutton, switch using digitalRead() function.

To avoid this, beginners SHOULD pay attention to the following issues:

1. Floating input problem:

2. Chattering phenomenon

It should be considered in only some application that needs to detect exactly number of the pressing.

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