Laughing in Class

Have you ever been sitting in class and think of something that has recently happened, and just started laughing? This happens to me, ALOT! For instance I recently watched this YouTube video: http://www.youtube.com/watch?v=-QbZ12WRLL0. Now when ever I am in class I just imagine random people doing standing up and running out of the room in a superman costume. Whats worse is, in the psychology class I take this happens to me the most. Yesterday we were talking about depression and such and I just burst out laughing. Now I have to try to explain to my teacher why I think this topic is so funny.

Now, this doesn't always happen in aspects of a YouTube video. Sometimes I just daydream and imagine funny things happening, like a sarcastic answer to a question the professor asks or my best friend tripping over a book about baby names and unknowingly exclaiming "Oh, Baby!" (that might be a 'you had to be there' moment)

What I want to know is if this ever happens to any one else and what exactly are the circumstances? Also, what do people suggest one do to calm these sudden urges to break out in spontaneous laughter?
Peace