Explain the basic idea of supply and demand -- prices tend to rise when something is scarce and popular, and fall when it is plentiful -- using a farmers market scenario.
Supply is how much of something is available. Demand is how much people want it. When demand is high but supply is low (something scarce and popular), the price usually goes up. When supply is high (plenty available) and demand stays the same, the price usually goes down.
A farmers market is a great place to see this in action, because prices really do shift with the seasons. Early in strawberry season, when only a few farms have ripe berries, a basket might cost more. At the peak of the season, when many farms have plenty of ripe strawberries, the same basket often costs less, because supply has caught up with demand.
Mastery looks like: They can explain supply and demand in their own words and predict a price change from a simple scarce-or-plentiful scenario.
Common stumbles: Mixing up which direction price moves for scarce versus plentiful goods.