This strikes me as funny in the sense of peculiar (not necessarily "Ha-ha funny"). That's because I never heard of a dog having just one puppy at a time. Don't they most always have considerable litters of puppies at a time? Incidentally there is a site called iusedtobelieve.com on which people post their mistaken childhood beliefs. One of the most common beliefs thereon is that dogs are males and cats are females of the same species. So sometimes or others children must have reacted to this line with "What? Dogs don't have puppies! Cats have kittens AND puppies!"