Sentence 1: I have an old dog. 
 Sentence 3: I want to have an active dog 
 Sentence 4:  One that I can take on hikes with me 
 Sentence 5:  I think I need to get a new puppy 
 Option 1: His hips hurt, so he can't do much. 
 Option 2: My dog's hips hurt him too much, so I decided to put him down.