Sentence 1: Mary was dog sitting for a friend who was on vacation. 
 Sentence 3: Simon didn't realize that someone had left the gate open 
 Sentence 4:  When the dog saw the open gate, she took off running after a rabbit 
 Sentence 5:  Simon was able to catch her when she stopped to sniff a fire hydrant 
 Given Sentence 2: One day, Mary's son Simon was playing in the backyard with the dog.