const int lowestPin = 2;
const int highestPin = 4;
void setup() {
  //set pins 2 through 13 as outputs:
  for(int thisPin = lowestPin;thisPin <= highestPin;thisPin++){
    pinMode(thisPin,OUTPUT);
  }
}
void loop() {
  //iterate over the pins:
   for(int thisPin = lowestPin;thisPin <= highestPin;thisPin++){
    //fade the LED on thisPin from off to brightest:
    for( int brightness = 0; brightness < 255;brightness++){
    analogWrite(thisPin,brightness);
    delay(2);
   }
 //fade the LED on thisPin from brightest to off:
    for( int brightness = 255; brightness >= 0;brightness--){
    analogWrite(thisPin,brightness);
    delay(2);
   }
   //pause between LEDs:
   delay(100);
   }
}