To clean mussels, rinse them under cold water and use your fingers or a brush to remove any dirt from the shell. Mussels have a very strong fibrous thread that they use to attach themselves to the substrates in the ocean. This thread is referred to as the "beard" and it needs to be removed before cooking . Using a paring knife, grab it between the knife and your finger and pull it toward the hinge of the shell until it releases. Many of the mussels will have released their beards already.