Eating stones

I have Louis the 8 months Frenchie with us. Yesterday took the dogs to pub they had lots of attention and were playing when I saw peppa trying to get something out of Lou's mouth I opened it and under his tongue was a big stone! It's not the first time and happened to owner as well why is he doing it??
He has very runny tummy as owner changed his diet from royal canin to Simpsons and I doubt he did it gradually.
I'm afraid to think what have happened if he did swallowed the stone . Any ideas why he does it and what to do?

Some dogs just do it, Lily, but it really isn't good. If a dog swallows a stone that's too big to work its own way out, it can mean an operation.

It reminds me of that incredible story of the GSD that had 28 golf balls in her stomach:

http://news.bbc.co.uk/1/hi/england/manchester/3652903.stm

How could you not notice the volume that would take up, I mean surely the stomach would be swollen and lumpy?

The there's the dog that ate the fairy lights. And the puppy who likes eating glasses (that one's called Millie...). Why they do it? Goodness knows!

I dnt know why they eat stones tbh may be just a habit they have gotten into .