Prior to cooking live mussels, discard any that appear chipped or broken. Fresh mussels may also have remaining Byssus threads, or “beard”, which they use in the wild to anchor themselves to a growing medium that should be removed before preparing. If the mussel is slightly open, give the shell a light tap. The shell will close partially if the mussel is alive, as only living, clean mussels are fit for cooking. There are multiple options for cooking mussels, but regardless the method, they will be fully cooked when the shells have opened approximately 1/4 inch.