For people new to using the Arduino IDE with various hardware, it might be worth pointing out that while a pullup is needed, and the Atmega328 has internal pullups when configured with INPUT_PULLUP, ...

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If you want a button or switch as an input on a microcontroller (MCU), you can use a “pullup resistor” so that the input is seen as a logical high when the (normally open) switch or button is closed.