But don’t just say “I don’t know” when you are stumped. You want to communicate that you are engaged and are eager to find out and learn. Saying something like, ...

DO say "I don't have the right words." It's OK to acknowledge that you don't know what to say, says Harris. Your friend will understand that it's hard to get the words right.

Based on the thousands of responses people answered with, the moral of this thread is: If you don’t know what to say to someone, your best bet is to zip it.