henry copeland:

Wriggly, giggle, puffball: What makes some words funny? Researchers are cracking the science of humor, one word at a time -- ScienceDaily

"We started out by identifying these six categories," said Westbury. "It turns out that the best predictor of funniness is not distance from one of those six categories, but rather average distance from all six categories. This makes sense, because lots of words that people find funny fall into more than one category, like sex and bodily functions -- like boobs."
- www.sciencedaily.com