Originally Posted by Dovster

As an American citizen living outside of the country, I was thrown by that question the first time I was asked it.

I replied that my trip was business and then asked what kind of business I would be doing in the States. I answered that I work outside of the country and thus the trip (US-Israel-US) was a business trip, even though my current itinerary was Israel-US-Israel.

I learned after that just to say "pleasure" and get through with less hassle.