The Sleepy Dog<br>The dog laid down beside her, yawning. He was obviously tired from playing all day. He curled up in a ball so he could sleep. He flopped onto his back after a while. He just laid there, waiting for a tummy rub.
The dog had never had a tummy rub before, which is why he was waiting for one.
If the dog had never had a "tummy rub" he wouldn't know to roll on his back to get one.