Mussels - how open is open.

[LimeSlice]

With mussels, if you can push them closed very lightly and they close and stay closed totally, then they are fine. You should be able to feel them close themselves, as opposed to just closing because you closed them...sounds odd but if you do it you will feel what I mean. They will often open and close if you bought fresh ones, depending on how they have been stored.
Open is open though so much more than a mm and I would bin it

ETA: are you meaning it is barely open after cooking? lol Sorry, I thought you meant before you cooked it .