const int lowestPin = 2;

const int highestPin = 13;

void setup() {
 
 for (int thisPin = lowestPin; thisPin <= highestPin; thisPin++){
  
  pinMode(thisPin, OUTPUT);
 
 }

}

void loop() {

  for (int thisPin = lowestPin; thisPin <= highestPin; thisPin++){
    
    for (int brightness = 0; brightness < 255; brightness++){
      
      analogWrite(thisPin, brightness);
      
      delay(2);
    
    }
    
    for (int brightness = 255; brightness >=0; brightness--){
      
      analogWrite(thisPin, brightness);
      
      delay(2);
  
    }

    delay(100);
  
  }
}