Has anyone ever experienced walking alone somewhere (or being on the train) and being deeply in one's thoughts, then suddenly remembering something funny and bursting out laughing - much to the bewilderment of everybody else around?

It happens to me all the time - I tend to think about all sorts of things when I go somewhere, and if I remember some funny situation then I can't help laughing. It's a dilemma really because I don't want people around me to think that I am "crazy", so I end up pretending that I'm trying not to sneeze. It makes the laughing fit only worse though because pretending to try not to sneeze is funny in itself.

What is one supposed to do - when going somewhere alone, and having nobody to talk to, I have to think about something. And sometimes I just think of some funny situation. The only problem is that all the other people around me always look miserable so it stands out of course if I am smiling.

Does anyone else experience that?