While riding the subway in her hometown of Barcelona, Eliana Guerrero saw pickpockets steal a case of insulin from two elderly tourists. That crime motivated Guerrero to do something for help her city. “I try to solve things that affect me directly,” says Guerrero. “Pickpockets directly affect me because I adore Barcelona.”

Since 2009, Guerrero has spend about three a hours a day patrolling Barcelona’s subways looking for pickpockets. “My mother always told me, ‘One swallow doesn’t make a summer. Many people are needed to change things,'” says Guerrero. “But I don’t think so. If someone says somethings wrong here, that’ll draw attention and more people will eventually join you.”

Watch how one woman armed only with a whistle, pepper spray, and determination is helping to clean up crime in her city.