Why did my teacher smile like this?

Although he is friendly and chatty, quite the gentleman etc, he has a very dry sense of humour and is oh so serious and VERY level headed. He is definitely not one of those people that will laugh at every little thing, he is selective in what he finds funny, but he isn’t strict or anything.

What is bothering me is that on several occasions now when I have said something to him, perfectly ordinary, he smiles in that way people do when they are trying not to laugh. Like a suppressed smile. And he doesn’t want me to see it, it is sort of to himself.

For example I told him I would give him my work at lunch and he did that dumb smile before saying that that was fine.

Then most recently in the hallway I asked him about an article for the school magazine and he did that stupid smile again, so much so I was about to ask him just what the hell was so funny (I wasn’t supposed to see it as he tried to walk around me while doing it, but I was looking right at him), he then turned to face me and told me that he would have the article done by tomorrow in all seriousness.

• He is not of a nervous disposition, he is very calm and controlled.
• On all the occasions, up until I said something to him he was always serious, as in he cannot have been laughing at something else when I happened to talk to him. It was always because I had said something to him.

I never see him do this when other people in my class say things to him. It bothers me as I feel like I must be doing something without realising. Any insight into why he is doing this? If he was generally a jolly jokey person I wouldn’t have taken any notice, but he isn’t, I wasn’t trying to be funny. Help?