I recently asked two of my professors to write me recommendation letters and they promptly and kindly accepted to do that. It is now two weeks since then. I want to send “thank you” notes to them. Is it enough to send e-mails? I have heard that it is more polite to send a written letter by US post, but it is a little bit weird when we are all in the same building! Isn’t it? Can I send them some kind of gift on a special occasion?