Based on the information present in the given passage, you need to write a correct answer to the given question. A correct answer is one that correctly and completely answers the question. Do not write answers that address the question partially or incorrectly. The correct answer must be contained in the given paragraph.
Q: Paragraph- Sent 1: Lucy is an eight year old girl who lives with her parents and pet bird.
Sent 2: One Saturday afternoon, she sits at the kitchen table.
Sent 3: She draws a picture of her family.
Sent 4: She makes sure to draw her mom named Martha wearing a purple dress, because that is her favorite.
Sent 5: She draws many yellow feathers for her pet bird named Andy.
Sent 6: After she is done, she gives the picture to her mom.
Sent 7: Her mom hangs it up on the silver refrigerator.
Sent 8: The next day, Lucy's mom comes home with a brown dog named Oliver.
Sent 9: She tells Lucy that she is pet-sitting, which is like babysitting but with a friend's dog.
Sent 10: Lucy's mom tells her she can play with the dog as long as she takes good care of him.
Sent 11: Lucy is excited.
Sent 12: She pets Oliver nicely on his soft back.
Sent 13: He wags his tail.
Sent 14: He barks, "Woof!"Sent 15: and shows he wants to play fetch with her.
Sent 16: Lucy giggles and throws a ball for him to catch.
Sent 17: She wishes her bird could play like this with her every day!. 
Question: What makes Oliver wag his tail?.
A: 
Lucy pet him.