In case you were wondering, the joke comes from Yoram Bauman "The Stand-Up Economist" via "econgirl" of Economists Do It With Models (facebook: Jodi Beggs), who I gather is an economics grad student at Harvard. I thought of all the "in bed"s.

“You might be an economist if you go to a Chinese restaurant and open the fortune cookie and add ‘at the margin’ at the end of it.”

http://www.economistsdoitwithmodels.com/2009/03/13/the-ten-principles-of-economics-by-the-stand-up-economist/