/**
  * Feed the item to a specified pet.
  *
  * Increases pet's hunger level and destroys the item. 
  * 
  * @param Pet $pet 
  * @return string The success message. 
  **/
 public function feedTo(Pet $pet, $quantity)
 {
     if ($quantity > $this->getQuantity()) {
         throw new ArgumentError("This stack does not have {$quantity} items.");
     }
     // Do it before destroying the object.
     $pet->consume($this->getHungerBonus() * $quantity);
     $text = "You have fed {$pet->getPetName()} {$this->makeActionText($quantity)}.";
     $this->updateQuantity($this->getQuantity() - $quantity);
     return $text;
 }