Laughing at Nothing

In the Screwtape Letters by C S Lewis in one of the chapters he talks about how laughing is done at various times. His four were:
Joy (like when you meet a friend of family after a long separation).
Fun (when you are just enjoying yourself)
Joke Proper (Jokes and incongruities)
Flippancy (in a sense taking everything serious and otherwise to be funny)
Yesterday I was in a car for something like 8 hours. With one real adult, me and another college graduate and four teenagers. Two whom I knew vaguely, two of whom I knew well last year at this time but haven't really talked to since, and two of whom I didn't know at all.
Between the sleeping and the eating and the minecraft on tablets. And the stopping. We laughed a lot. When we stopped at a restaurant I think the waitress thought we were all crazy. Good thing there wasn't many people there. What kind of laughter was that? Perhaps a little of all four. It was joy to see the people I knew again. Fun to laugh at/with some of the comments of the teenagers, plus quite a few incongruities going on. . . But honestly maybe C S Lewis should have added another category. Doesn't slap happy or this is taking a long time and we are all tired count as a fifth category... Because I'm not sure we were even laughing at incongurity towards the end... it was more like laughing at nothing.