Why are there cars?

On a drive home my son noticed these posts with little metal horse heads with rings on them and asked, "What are those horses with rings for?" I explained that a long time ago people used to ride horses and they would tie up the horse there so they wouldn't run away while they were inside. The he asked "Where are the horses?" I explained that most people drive cars now so that is why you don't see any horses, and then he had the followup "Why are there cars?" I was now caught in my own trap of explaining something. Then I explained that a long time ago a guy invented the gasoline engine so people could get places quicker. "Is that like a steam engine?" he asked as he is really into trains. "No it is not, it is more like a diesel train engine," I proceeded to explain and that he understood. "So it makes smoke like Diesel when he crashes?" At that point after a lot thinking and explaining I found a way out... "Yes!" He then giggles and says, "Silly Diesel!"