How to stop grinning in serious/nervous situations?

Hi, I have this problem with grinning in serious situations even when I'm nervous, scared or upset, and it makes me come across as rude and flippant. Anything not obvious? I always cover my face/mouth, but I think it still looks obvious. Don't say suck on a lemon, because it usually happens randomly, and I don't have the time to get a lemon obviously. lol.