user
Given a cartoon image and a caption. Caption: If their dog starts humping your leg - let it finish. Question: Generating an explanation of why this image/caption combination is funny. Answer: 
assistant
A crude joke about the enormous dog not pictured, but implied to exist. People often tie their dogs up with chains, but the chain here is enormous and powerful, suggesting the dog its attached to is very large and strong. So, if it were to start humping your leg (normally, one would push a dog doing this away), it's safer to just let it continue rather than anger it.