How to make a cup of tea =)

make_tea.php

<?php

$cup = new Cup(250);
$spoon = new Spoon();
$someWater = new Water(250);
$sugar = new Sugar();

$cup->put($sugar);
$cup->put($spoon);
$cup->poor($someWater);
$spoon->stir();

// ---------------------------------------

class BaseObject
{
    private $children;
    private $parent;

    public function put(BaseObject $object)
    {
        $this->children[] = $object;
        $object->parent = $this;
    }

    public function getChildren()
    {
        return $this->children;
    }

    public function getParent()
    {
        return $this->parent;
    }

    public function move()
    {
    }
}

class Food extends BaseObject
{
}

class Sugar extends Food
{
}

class Water extends BaseObject
{
    private $volume;

    public function __construct($volume)
    {
        $this->volume = $volume;
    }

    public function getVolume()
    {
        return $this->volume;
    }
}

class DinnerObject extends BaseObject
{
}

class Spoon extends DinnerObject
{
    public function stir()
    {
        parent::move();
        $parent = $this->getParent();
        if ($parent instanceof Cup) {
            if ($parent->isFilled()) {
                $children = $parent->getChildren();
                $canStir = false;
                foreach ($children as $child) {
                    if ($child instanceof Sugar) {
                        $canStir = true;
                    }
                }

                if (!$canStir) {
                    throw new Exception("There is no sugar!");
                }
            }
            else {
                throw new Exception("The cup is not filled!");
            }
        }
        else {
            throw new Exception("Can't stir");
        }
    }
}

class Cup extends DinnerObject
{
    private $volume;
    private $filled;

    public function __construct($volume)
    {
        $this->volume = $volume;
        $this->filled = false;
    }

    public function isFilled()
    {
        return $this->filled;
    }

    public function poor(Water $water)
    {
        if ($water->getVolume() >= $this->volume) {
            $this->filled = true;
        }
    }
}