Looking at videos of myself memorising cards has shown me that I fiddle with my beard when I'm concentrating. I had absolutely no idea I did that. It does explain why last week at work when people were talking about some obscure subject and I was wondering whether to jump in with my huge store of trivial knowledge or just get on with my work, a colleague said "Ben's stroking his beard, that means he's got something to say..."
Obviously it's a thing that I do. I bet it's a real giveaway when I'm playing poker, too.