fasterXML Jackson 2.2.3. How did it figure out outer XML markup

I’m developing REST service and I used Jackson 2.2.3 library from fasterXML (http://wiki.fasterxml.com/JacksonHome). I want to get a list of activities in XML. This is POJO of that activity decorated with Jackson anotation:

@XmlRootElement
public class Activity {
    private String description;

    public String getDescription() {
        return description;
    }
    public void setDescription(String description) {
        this.description = description;
    }
}

Now I put few activities in the list and what I get is XML structure:

<activities>
  <activity>
    <description>Swimming</description>
  </activity>
  <activity>
    <description>Running</description>
  </activity>
</activities>

now the question is how did Jackson library figured out the outer markup name? I mean how did it get word activities? My code doesn’t have such word!