What's the difference here?

I realized, when trying to explain it to a friend a week ago, that while I can correctly select between "which" and "that" in context, I can't enumerate rules.  In interpreting language, I think nearly everyone can tell that there's a difference.  To illustrate, two sentences with clearly different meanings:

"I feel an intense need to do something that is challenging."
"I feel an intense need to do something, which is challenging."

Here's a formal description of the difference (with appropriate skepticism of any difference between the two).

Posted via email from Ben Weinstein-Raun