How Timothy Leary Adds Value to an Unopened Envelope

Friday fun link: Acceler8or interviews William Gibson about the cyberculture/counterculture of the 1990s. My favorite part involves Timothy Leary:

[P]eople would come up to him and give him drugs, which he'd give to someone else, usually a perfect stranger, as soon as the gifter was gone. He said that this was a win-win proposition, as the first person could now say that he'd given drugs to Timothy Leary, and the second person that Timothy Leary had given him drugs. I never saw him look to see what was in the envelope.

Question for the commenters: Suppose you're an economics professor who wants to tell that story in the classroom. What principle would you use it to illustrate?