5th wheel

I remember the time when someone asked me this "how many wheels does a moving car has?". I answered, "of course it is four!" Then, he said "no, you are wrong". I was little then and of course I replied, "why am I wrong". Then he explained to me that a moving car should have five wheels. The fifth wheel is the spare wheel once one tire became flat. So until now I am thinking that a 5th wheel warranty is very essential. We are not always experiencing flat tire at the road so there are instances that the 5th wheel rottens without us using it, right?