Good job on the tests! I would update this though, just so that we are not using `->set()` to put the item in the container, so that we are testing it in isolation. Right now, _if_ the `set` method broke for whatever reason, it would make this test fail as well.

```php
$reflection = new \ReflectionProperty($this->fixture, 'dataStore');
$reflection->setAccessible(true);
$reflection->setValue($this->fixture, array('foo' => 'bar'));

$this->assertTrue(
    $this->fixture->exists('foo'),
    'When calling exists on an item that has been set in the container, it should return true.'
);

$this->assertFalse(
    $this->fixture->exists('baz'),
    'When calling exists on an item that has not been set in the container, it should return false.'
);
```