Fresh mussels must be bought live — dead mussels can be toxic. Tip the mussels into the sink and sort through them — they should all be closed. If any shells are open, tap them on the sink. If they stay open, throw them away, as they are dead. Throw away any other mussels with cracked shells. Any mussels that don’t open during cooking are also dead and should also be discarded.
Scrub the mussels thoroughly under cold running water to remove any dirt or slime.
If the hairy beard is still attached, pull it off, then rinse again.