I've been priming Soren for this for months by telling him a bedtime story about how Mommy and Soren make a snowman. In the story, after each step, Mommy says, "Now the snowman is done!" And Soren says, "No, Mommy! It's not done! It still needs______(a nose, a hat, buttons, eyes, arms, etc.) And Mommy says, "Oh! A _________!" And they give the snowman whatever it is that it needs.
Much to my delight, Soren incorporated the story into our actual snowman-building experience. We would put in the snowman's eyes and he would ask, "Now what does the snowman need?" And I would reply, "I don't know. What does it need?" And he would say, "It needs a hat!" And we would get a hat.
At the end of the bedtime story, Mommy is pretty certain that the snowman is done, but Soren says that it isn't. "Soren," says Mommy. "Our snowman has a hat. He has a nose. He has eyes. He has a mouth. He has two arms. He has buttons. What else could he possibly need?"
And Soren says, "He needs a hug!" So he gives the snowman a hug.