In a 1984 episode of Sesame Street, Prairie Dawn, the most-level headed Muppet, is faced with a difficult challenge: she must explain the concept of ‘the last cookie’ to the Cookie Monster. The presentation begins well. Prairie asks the Cookie Monster to eat three of the four cookies on the table, and he does so with typical enthusiasm. But the mood changes when Cookie Monster is faced with the last cookie.

“But Prairie! Prairie! That’s the last cookie! After we eat that...