The fluidity of caramel is determined by water content. Are you familiar with the cooking stages of sugar - thread, soft ball, soft crack, hard crack, etc? If you cook sugar and water together, as the water evaporates the temp goes up and the finished product is harder. The water in a soft caramel or caramel sauce comes from cream or milk; if you cook it all away you will have hard candy. Or if you cook sugar and butter to caramel, it will be a hard toffee not a sauce.