Instructor asks the class how many students would be willing to pay various prices for a specific good. Results are tabulated into a demand curve. Extensions include changing the conditions to shift the curve (e.g., would demand for a cold soda change if the day were hotter/colder).
or larger classes, can be done with a sub-set of students as a demonstration for the rest of the class, or results can be collected with clickers.
Salemi, Jee 2009, describes similar activity but asks students to name their price (i.e., runs an auction) for a unique good, rather than naming price and tabulating quantity. This focuses attention on reservation prices.