Rinse the mussels under the tap, scrub the outside of the shells to clean and trim the 'hairy' beard.Knock any open mussels hard with an implement (spoon/knife/fork). If they don't shut throw them out. Then wash again.
There is one simply rule with mussels; when they are uncooked throw away any open mussels, when they are cooked throw away any that stay closed.
As with any shellfish take particular care and attention in the cleaning and preparation of your mussels, if you have any concerns at all always check with your fishmonger or shellfish supplier.
Melt the butter gently in a pan large enough (with a lid) to hold the mussels.
Serve straight from the pan into large bowl. Serve a bowl of melted butter separately for dipping the mussels in.