I have a short attention span, and when I'm in a business setting, or some other serious situation, I often suffer this horrible problem. My mind will tune out whoever is talking in a meeting, and I'll begin to entertain myself with my own thoughts. Sometimes I'll think of something that I find especially humorous, and I'll have a burning desire to chuckle and snort about it. But there's no way I can laugh.
Imagine your manager looking you directly in the eye and explaining to you the seriousness of a situation at work. Perhaps there's a crisis that you're discussing. And imagine that in the middle of this, you can feel your face turning bright red as your cheeks get rosy and puffy. You're fighting the urge to laugh so hard that you're only making it worse. And the thought is something you can't possibly explain. Perhaps it's the realization that one of the VP's of the company's name rhymes with Babaar, that elephant king from the children's books you read as a kid. Or you realize that a drawing on the whiteboard in the conference room looks a little obscene. Whatever.
When I'm in this situation, which I'm in quite often, I quickly try to come up with a comment I can make that's relevant to the business conversation but is also a little humorous. I imagine it's better to be abnormally amused with a benign joke and avoid revealing my short attention span than to admit to my boss that the ideal porno name for him just occurred to me, and it's cracking me up inside.