Real answer: I usually say thank you in person at the end of the interview. Like "Thank you for your time." Sending an email restating that would be wasting their time, no?
Nah, these people stir air for a living. They appreciate others who also know how to stir air. Sending a pointless time wasting email is the epitome of this discipline. If you want in, you need to show them you can do it too. If you want to be an actual productive member of society, you wont do it and find a position where you don't have to resort to such menial tasks.