Button on pin 12 lights the LED on pin 13, on an Arduino Uno

// A button on pin 12 lights the LED on pin 13.
//
//   one side of the button -> 5V
//   the other side -> pin 12, and a 10k resistor from pin 12 to GND
//   the LED is the Uno's own, on pin 13
//
// The 10k is a pull-down: with the button up it holds pin 12 at 0 V, so the
// pin reads LOW instead of floating. Pressed, the button joins pin 12 to 5 V
// and it reads HIGH. (On the bench a 0.1 uF capacitor across the resistor
// smooths out the contact's bounce.)
//
// Hold the button on the canvas and watch the L LED on the board.

const int BUTTON = 12;
const int LED = 13;

int last = LOW;

void setup() {
  pinMode(BUTTON, INPUT);
  pinMode(LED, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: hold the button on pin 12");
}

void loop() {
  int now = digitalRead(BUTTON);
  digitalWrite(LED, now);
  if (now != last) {
    last = now;
    Serial.println(now == HIGH ? "pressed: LED on" : "released: LED off");
  }
  delay(5);
}