"How can I be certain that we're all thinking the same thing when we use certain words?" is a personal favourite.

*nod* This is why I tend to throw lots of words at concepts, especially if I think it's likely to mean different things to the people that I'm talking to. It's also why I like people who echo back what they understood me to be saying! (because it's usually in their own words, and not mine, so I can see if the translation happened correctly)