Never eat mussels whose shells are cracked, open, or any that refuse to close their shells when you handle or tap them, those are likely dying or dead. And be sure that they smell like the ocean, briny and like sea air, not fishy. Try to cook the mussels immediately (unwrap them as soon as you get home). If you have to wait, place them in a bowl and cover them with a damp towel so they can breathe.