to cast the sandwich

I spent much of December packing up my belongings and sending them off to storage; with the New Year I flew to Sweden to visit friends and do some travel. This evening I learned a delightful phrase "kasta macka", which means to "cast the sandwich", or to "skip rocks". However, in this case, the "sandwich" referred to is the flat, sedimentary rocks which have weathered into stones which are perfect for skipping across the surface of a body of water when thrown at the correct angle.