How does reply get added to correct thread in this test?

I'm currently revising the Lesson 4 of building a forum with TDD approach and wondering why the following test works [ Ref: https://github.com/laracasts/Lets-Build-a-Forum-in-Laravel/blob/76a05a038ae9a846df9b11ab1fcff48376ce74cf/tests/Feature/ParticipateInThreadsTest.php ]

In the following test, we create a thread and then create a reply and assume that the reply gets added to the correct thread.

Why does it work without having to write: $reply = factory('App\Reply')->create(['thread_id' => $thread->id]);

I think we need to specify the thread_id explicitly for the system to understand which thread to add the reply to.

Yet, everything works fine. I'm not sure why. Can someone spare a moment to explain?

/** @test */
    function an_authenticated_user_may_participate_in_forum_threads()
    {
        $this->be($user = factory('App\User')->create());
        $thread = factory('App\Thread')->create();
        $reply = factory('App\Reply')->make();
        $this->post($thread->path() . '/replies', $reply->toArray());
        $this->get($thread->path())->assertSee($reply->body);
    }