vango

Note the -n on vangojn after sur in the first example: motion onto a surface takes the accusative, rest on it does not. Larmoj fluis sur ŝiajn vangojn is tears running down onto the cheeks; larmoj estis sur ŝiaj vangoj is them simply sitting there. This is the single most useful thing the accusative does beyond marking objects, and cheeks are as good a place to drill it as any. A slap on the cheek is a vangofrapo, one of those transparent compounds Esperanto builds without ceremony.