Medium-sized clicky momentary switches are standard input "buttons" on electronic projects. These work best in a PCB but can be used on a solderless breadboard as shown in this tutorial. The pins are normally open (disconnected) and when the button is pressed they are momentarily closed.
They come in a pack of 10, they're pretty handy so you'll want to keep them around.
Although it is Chinese, but also very easy to understand。
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