Ah of course! Leave a textbook open, or a notebook open that shows you were in the middle of a sentence before you had to go take a call. I've actually thought

Ah of course! Leave a textbook open, or a notebook open that shows you were in the middle of a sentence before you had to go take a call. I've actually thought about doing an experiment with an iPod: I'd leave an iPod in the student union and observe how long before someone turns it in / tries to run away with it.