You've probably seen those little cards in hotel rooms: towel on the floor if you want a fresh one, towel on the rack if you'll use it again. And using towels more than once means less laundry, which saves water and energy. But you don't pay the utility bills at the hotel. So, do you really care? Well: a new study in the journal Nature Energy suggests hotel guests really are motivated to conserve - for factors other than money.