Mussels attach themselves to stable surfaces using thin, sticky, weedy membranes referred to as “beards,” which must be removed before cooking. Most farm-raised mussels purchased in grocery stores have already been debearded. If the beard is still attached, grasp it with your fingers and pull it downward toward the hinged end of the shell. Pull firmly until it comes out and discard it. Place mussels in a colander in the sink, and run cold water over them to get rid of any visible dirt or grit on the outer shells. Using a vegetable brush, scrub each shell under running water. Overlooked grit can ruin a dish.