Externalities are usually exemplified by tales of a company forcing citizens to bear some of its costs, for example, a polluter dumping its waste into a river and leaving it to the community to pay for clean-up. In a role reversal, many parents are dumping their teens at the mall rather than bear the burden of looking after the little darlings themselves.
The Christian Scientist Monitor reports  that malls are attempting to cut back on rowdy behavior and loitering, which they believe leads to lost sales, by enforcing policies that require teens to be accompanied by parents after certain hours.
From the Monitor: "Mall managers say that packs of teens deter families from choosing the mall as their Friday night outing. Many complain they have become de facto baby sitters. "
 At shopping malls, teens' hanging out is wearing thin, Sara B. Miller, Christian Science Monitor, August 11, 2005, http://search.csmonitor.com/search_content/0811/p01s01-ussc.html