Mussels cook quick and taste great with little prep, but cleaning and debearding them is crucial for the cooking process.
Much like clams, mussels are cooked live. Purchase mussels that are tightly closed and smell like the sea. If any are open before cooking them, tap them — if they close within 30 seconds, they're still alive. If they don't close, discard them. Also throw away any that don't open after cooking.
Like most fresh shellfish, mussels may still have sand or barnacles on them, so wash them well before cooking. Many may also have a bristly thread called a beard, which helps anchor them to rocks in the ocean. To remove it, grasp it with your fingers or pliers and pull it toward the small end of the shell — the beards should detach easily. Just be sure to wait and remove the beards until just before serving, so the mussels don't die in the process.