Why put a stone in the soup?
There's an old children's story about making Stone Soup. In it, a penniless begger offers to teach people how to make his favorite recipe: soup, made from a stone! He boils some water and drops a stone in, and while it's "cooking", keeps mentioning offhand things like "It'd go great with some carrots" or "Celery would be lovely in this". The townspeople rush off and get celery, carrots, potatoes, onions, and the like to add to the soup, until eventually, they've made proper soup in his pot along with the stone. So basically, the whole "stone soup" thing was a clever con game by which the begger can eat for free (or, in kinder versions than the one I recall, to trick the townspeople into learning a lesson about sharing). 

The stone adds nothing to the soup.