I Dont Know
Probably the hardest thing for me to say. I'd rather BS my through an answer or contribution instead of saying I don't know so I can learn.
There are people who will mock you for not knowing things, but I think that these people fail to see the broader world and the possibility of things to know. They are not curious people and will deride you just as much for knowing "useless" things as they will when you don't know actually useless things, like who is the quarterback for such and such team.
But a friend told me recently how he's trying to increase the number of times he says I don't know, especially when feel like you "should" already know something. It opens the doors to learn.