demand
Explanation
This comic shows an economics professor leaning out a window shouting "Free sex! Absolutely free sex with an economics professor!" The caption reads: "Professor Rosen conclusively demonstrates that the demand curve does not go to infinity as price approaches zero."
The joke is rooted in basic microeconomics. The law of demand states that as price decreases, quantity demanded generally increases. Taken to its logical extreme, one might assume that making something completely free would generate infinite demand. Professor Rosen is testing this hypothesis using himself as the product -- offering "free sex with an economics professor" and discovering that, despite the price being zero, demand is clearly not infinite. Nobody is taking him up on the offer.
The humor works because the professor is simultaneously proving a legitimate economic point and humiliating himself. The comic highlights that demand depends on more than just price -- it also depends on the desirability of the product itself. An economics professor desperately shouting from a window is not an attractive proposition regardless of the price point. The comic also pokes fun at the stereotypical lack of social awareness among academics, and the tendency of economists in particular to treat everything as a market transaction, even romance and intimacy.