The wall behind Billy's bed is covered in clouds. We're going to say they're cirrus clouds, but we're no cloud experts. (If you ever see a wall decorated in pizza slices, though, feel free to give us a call. We can identify 478 different species of pie.)

We're told that Joanna painted those clouds, and while she didn't intend it, they're a symbol of home. In fact, they're such an important representation of home that they're the catalyst for Joanna's big change of heart at the end of the film. Check it out:

JOANNA: I woke up this morning. I kept thinking about Billy, and I was thinking about him waking up in his room with his little clouds over him that I painted, and I thought I should've painted clouds downtown because…

She trails off, sniffling.

And then he would think that he was waking up at home.

She sighs.

I came here to take my son home, and I realize he already is home. 

Those are some important clouds. They stand for security and stability, and they're the spark that finally makes Joanna understand that Billy has those two things with Ted.

Ted couldn't make her understand that. Neither could Margaret's impassioned plea from the witness stand in court. Nope; in the end, it's about four bucks' worth of white paint that convinces Joanna that the best thing for Billy is staying put.