private int timer;
private int chance = 125;
private void spawnAlien()
{
timer = (timer+1)%1500;
if (timer == 0) chance = chance-10;
int rand = Greenfoot.getRandomNumber(150);
if (rand >=3) return;
if (rand == 0) addObject(new Alien(), 870, 453);
}
public 
