Blink Adafruit_NeoPixel
The first argument of setPixelColor is is the number of the led. In your case 0, so the first led.

Instead use a for loop to set all the leds from 0 until (excluding) 24 (which is stored in NUM_LEDS).

void loop()
{
  for (int led = 0; led &lt; NUM_LEDS ; led++)
  {
    strip.setPixelColor(led, 255, 255, 255);
  }
  strip.show();
  delay(1000);

  for (int led = 0; led &lt; NUM_LEDS ; led++)
  {
    strip.setPixelColor(led, 255, 255, 255);
  }
  strip.show();
  delay(1000);


To make the code better (shorter, but more important more maintainable, and reducing code duplication), create a separate function:

void loop() 
{
  setLeds(255);
  setLeds(0);
}

void setLeds(int intensity)
{
  for (int led = 0; led &lt; NUM_LEDS ; led++)
  {
    strip.setPixelColor(led, intensity, intensity, intensity);
  }

  strip.show();
  delay(1000);
}